Add support for multiple accounts in config | Fix #98

Add four new flags

* -a | --account - Use a different account than default

* -la | --list accounts - List all the accounts configured with CLIENT_ID, CLIENT_SECRET and REFRESH_TOKEN

* -ca | --create-account - Create a new account

* -da | --delete-account - Delete an existing account

--list-accounts, --create-accounts and --delete-account flags can be used without any usual input required

The config format is changed, ACCOUNT_${account}_ prefix is added to every config var, where ${account} is the account name

The old configs will be automatically migrated with account name default( if default taken then a number suffix will be added like default1, default2, etc )

move authentication/config related functions to auth-utils.[bash|sh]

create more authentication functions to reduce some clutter

add a lot of error checks, minute situations like chmod failing, failing to write to config file

bash script changes: export all sub functions in their sub scripts

export variables when it is created that are needed for parallel uploads

README.md

@@ -27,6 +27,7 @@
   - To anyone or a specific email.
 - Config file support
   - Easy to use on multiple machines.
+  - Support for multiple accounts in a single config.
 - Latest gdrive api used i.e v3
 - Pretty logging
 - Easy to install and update
@@ -359,16 +360,18 @@ To change the default config file or use a different one temporarily, see `-z /
 This is the format of a config file:
 
 ```shell
-CLIENT_ID="client id"
-CLIENT_SECRET="client secret"
-REFRESH_TOKEN="refresh token"
+ACCOUNT_default_CLIENT_ID="client id"
+ACCOUNT_default_CLIENT_SECRET="client secret"
+ACCOUNT_default_REFRESH_TOKEN="refresh token"
 SYNC_DEFAULT_ARGS="default args of gupload command for gsync"
-ROOT_FOLDER_NAME="root folder name"
-ROOT_FOLDER="root folder id"
-ACCESS_TOKEN="access token"
-ACCESS_TOKEN_EXPIRY="access token expiry"
+ACCOUNT_default_ROOT_FOLDER_NAME="root folder name"
+ACCOUNT_default_ROOT_FOLDER="root folder id"
+ACCOUNT_default_ACCESS_TOKEN="access token"
+ACCOUNT_default_ACCESS_TOKEN_EXPIRY="access token expiry"
 ```
 
+where `default` is the name of the account.
+
 You can use a config file in multiple machines, the values that are explicitly required are `CLIENT_ID`, `CLIENT_SECRET` and `REFRESH_TOKEN`.
 
 If `ROOT_FOLDER` is not set, then it is asked if running in an interactive terminal, otherwise `root` is used.
@@ -382,13 +385,14 @@ A pre-generated config file can be also used where interactive terminal access i
 Just have to print values to `"${HOME}/.googledrive.conf"`, e.g:
 
 ```shell
-printf "%s\n" "CLIENT_ID=\"client id\"
-CLIENT_SECRET=\"client secret\"
-REFRESH_TOKEN=\"refresh token\"
-" >| "${HOME}/.googledrive.conf"
+printf "%s\n" '
+ACCOUNT_default_CLIENT_ID="client id"
+ACCOUNT_default_CLIENT_SECRET="client secret"
+ACCOUNT_default_REFRESH_TOKEN="refresh token"
+' >| "${HOME}/.googledrive.conf"
 ```
 
-Note: Don't skip those backslashes before the double qoutes, it's necessary to handle spacing.
+Note: If you have an old config, then nothing extra is needed, just need to run the script once and the default config will be automatically converted to the new format.
 
 ### Upload
 
@@ -422,6 +426,38 @@ These are the custom flags that are currently implemented:
 
     ---
 
+-   <strong>-a | --account 'account name'</strong>
+
+    Use different account than the default one.
+
+    To change the default account name, do
+
+    `gupload -a/--account default=account_name`
+
+    ---
+
+-   <strong>-la | --list-accounts</strong>
+
+    Print all configured accounts in the config files.
+
+    ---
+
+-   <strong>-ca | --create-account 'account name'</strong>
+
+    To create a new account with the given name if does not already exists. If the given account exists then script will ask for a new name.
+
+    Note 1: Only for interactive terminal usage.
+
+    Note 2: This flag is preferred over `--account`.
+
+    ---
+
+-   <strong>-da | --delete-account 'account name'</strong>
+
+    To delete an account information from config file.
+
+    ---
+
 -   <strong>-c | -C | --create-dir <foldername></strong>
 
     Option to create directory. Will provide folder id. Can be used to specify workspace folder for uploading files/folders.