mediawiki.api.options: Add module for API action=options

mediawiki.api.options: Add module for API action=options

Implemented mw.Api#saveOptions to save user preferences.

If necessary, the options will be saved using several parallel API
requests. Only one promise is returned that resolves when all requests
are complete.

If a value of `null` is provided, the given option will be to reset to
the default value.

Any warnings returned by the API, including warnings about invalid
option names or values, are currently ignored. This basically means
that all requests will succeed (barring networks problems, internal
server errors and such).

Change-Id: Ia015898ca910923e00bc53f099b4e5631d6ad45c