Deduplication with optional fallback to default

[strump]

Added CLI argument --fallback-to-default.

For example in next strings 'es' should not be removed (fallback_to_default = False)

[bar]
en = Bar
es = Bar
uk = Bar

While in next sample all languages except 'en' should be removed (fallback_to_default = True)

[take_exit_number_street_verb]
en = NULL
de = NULL
nl = NULL

Those two different behaviours new CLI argument was added.