Read/write versionable FIG document

[poirierlouis]

Current FIG document formatter write JSON data where each key (widget property) is minified to reduce the total size of the file.

This proposal should define plain keys as widget properties and pretty print JSON.
It would make it easier for human reading and pretty printing would allow better support when versioning a FIG document.
Size of the document is not important as a FIG file is ~1MB when embedding a custom font.

It should be implemented after #47 is done.