Custom command

[augnustin]

Regarding this issue: https://github.com/elixir-editors/elixir-tmbundle/issues/175

Two options added:

• mix_format_command: allow to trigger a custom command instead of mix format only
• make_path_relative: which may be necessary if file is contenerized and path varies. I left it as an option as it is very naive and may not work on every environment (tested under linux)

Everything's backward compatible.

Hope it will be useful.

Best

[josevalim]

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