Add .editorconfig file

[iiegn]

We could probably add a .editorconfig file (to be explicit about some issues which would otherwise be 'only' written in something like a CONTRIBUTING.md file) - also see #112.

# EditorConfig is awesome: https://EditorConfig.org

# top-most EditorConfig file
root = true

# Unix-style newlines with a newline ending every file
[*]
end_of_line = lf
insert_final_newline = true
indent_style = space

# Matches multiple files with brace expansion notation
# Set default charset
[*.{js,py}]
charset = utf-8

# 4 space indentation (THE python way)
[*.py]
indent_size = 4

# Indentation override for all JS (to make call-back hell more readable)
[*.js]
indent_size = 2

[*.md]
trim_trailing_whitespace = false

# Tab indentation (no size specified)
[Makefile]
indent_style = tab

# Matches the exact files either package.json or .travis.yml
[{package.json,.travis.yml}]
indent_style = space
indent_size = 2