Add encoding support

Hi, 

Happy to find that package !
Since Atom 0.141.0, Encoding is supported, so it was easy to add this missing functionnality.

Hope you'll merge this pull request easily

Regards
You have to make sure the encoding keywords used by editorconfig is translated to what Atom uses though.

Example: https://github.com/sindresorhus/editorconfig-sublime/blob/26eed39f6b4943b8cc015e04932c1bd478c7aa54/EditorConfig.py#L24-L28 (don't use this ofc)