fallback-to-msgid option

If fallback-to-msgid is set, for those entries that would be omitted
(fuzzy entries without the fuzzy flag) and for those that are empty,
the msgid will be used as translation in the json file.
If the entry is plural, msgid_plural will be used for msgstr[1].
This means that this option makes sense only for those languages
that have nplurals=2.