Add additional supported fields

[jacobwegner]

I have a use case where I would like to store localized fields that are not CharField or TextField fields.

I'd like to start with adding JSONField, which seems to work pretty well.

I would also like to explore FileField. That field just stores a path in the database, but will need some additional work to wrap it so it returns FieldFile instance as expected.

[jacobwegner]

I started out with #119 and #120 after reading this in the docs:

Behavior is tested using CharField() and TextField(), as these make most sense for translated values. Additional fields could make sense, and will likely work, but need extra test coverage.

I have also considered an alternate approach to satisfying #118 that I could draft and submit as a PR. This approach would replace #119 and #120 by pushing out to project-specific configuration, e.g., providing MODELTRANS_ settings to customize:

• The supported fields (MODELTRANS_SUPPORTED_FIELDS)
• The TranslatedVirtualField class (which a subclass could be used to override the FileField functionality) (MODELTRANS_VIRTUAL_FIELD_CLASS)
• The TranslationField class (which a subclass could be used to override the pre_save functionality) (MODELTRANS_TRANSLATION_FIELD_CLASS)