Extend config to allow setting custom icon and highlight

[ttytm]

The PR extends the config and adds type annotations. With the update it will be possible to easily customize the sign via the config. It can also be a base for further extensions.

require("bookmarks").setup({
	sign = { icon = "" },
})

[LintaoAmons]

Hi Tt,

Thanks for making this PR, and I would love to add this function to the plugin.

I have some comments here, if you have some time can take a look, or I could do it later when I got some time~

1. I don't think we need make this name BookmarksNvimSign to be configurable.
2. Maybe makes the configuration more user understandable. Like:

{
  icon = "",
  icon_color = "red",
  line_font_color = "blue",
  line_bg_color = "red"
}

3. change the README accordingly

[ttytm]

1. I don't think we need make this name BookmarksNvimSign to be configurable.
   Good point about the BookmarksNvimSign, agreed .

2. Maybe makes the configuration more user understandable. Like:

{
  icon = "",
  icon_color = "red",
  line_font_color = "blue",
  line_bg_color = "red"
}

Personally, I would keep the config for the sign logically structured in a table instead of using a prefix. When extending the config it would could get unappealing quickly.

E.g., a feature I thought of is that it would be cool to be able to have multiple signs. E.g. when setting a mark without a note, and when annotating something with a mark.

I updated the PR be easily extensible for such potentials

3. change the README accordingly

Updated the readme. The commit includes an auto-formatting change. Let me know if this should be reverted.