Support .json files

[Spacerat]

This enables creating and editing .json files on CloudPebble, and also includes some related changed/refactoring. A detailed list of changes:

• .json added to the database validation regex for valid source file names
• .json included in places which check filename.endswith() (i.e. import/export code)
• Updated wscript as described by Clay's readme
• JSON option added to the new file dialog, which is also be available in pebblejs projects.
• Some refactoring in editor.js...
  • ... to use file_kind instead of the is_js flag. This is the same change made to support .monkey files in the test-bench branch.
  • ... in init_create_prompt() to factor out the repeated .then(...).catch(...) for each kind of file.
• Added json_parse.js which is used in place of JSHINT for .json files to get accurate real time json linting.
• Added two tests, for importing and exporting projects containing json files.

(Branched from #321)

[Katharine]

Geronimo!