Add support for the getAutonym callback

A getAutonym function can be passed as an option
when initializing ime, to give a possibility to load
autonyms of languages that don't have input methods
defined.

This is a step to resolving the following downstream bug:
https://bugzilla.wikimedia.org/show_bug.cgi?id=51025
Merge branch 'master' into bug-51025
Cleaner getAutonym()
Merge branch 'master' into bug-51025
Using getAutonym for all autonym getting
Merge pull request #265 from amire80/bug-51025

Add support for the getAutonym callback