JSON file support

Request for JSON file support in Zanata.

http://www.opentag.com/okapi/wiki/index.php?title=JSON_Filter

• This is support for JSON basic documents only, not the structured i18Next format.

• The filter allows the extraction of standalone strings, such as in arrays

• Documents can be uploaded to the File project type and translated

• Translations can be downloaded as filename.json via the WebUI, or pulled through client as ./locale/filename.json