Add support for a configuration file

[trixnz]

The Options object should be directly serializable to/from a .luafmt (or similar) JSON file to allow for persistent customisation per workspace.

[sum01]

Might I suggest having it respect .editorconfig settings, instead of implementing a specific .luafmt file? http://editorconfig.org/

[Stanzilla]

Yup, .editorconfig would be awesome! :)

[alanlivio]

Its will be very good an configuration file.

[AgentOttsel]

I would love to have a trailing comma option!