Write plural values in Twine format

[strump]

Python and Ruby implementations both support reading of plural values but not writing.

It means that Twine can parse file:

[bookmarks_places]
    en:one = %d bookmark
    en:other = %d bookmarks

but when we write it back we get only

[bookmarks_places]
    en = %d bookmarks

This PR fixes Twine exporter for Python. So after the fix we can parse Android and iOS plurals into Twine format. And even export plurals from strings.txt back to Android and iOS (not implemented yet).