Allow ignoring missing translations of blank source messages

[catrope]

For the requireCompleteTranslationLanguages and
requireCompleteTranslationMessages features, it doesn't make sense to
fail when a message whose source text is blank doesn't have a
translation. These messages aren't intended to be translated, and even
if they were translated as blanks, that would also fail because
disallowBlankTranslations is true by default.

This adds ignoreMissingBlankTranslations and sets it to true by default.
If enabled, a message is only considered missing if its source text is
non-empty. Blank source message do still require documentation.

[Krinkle]

This sounds good to me. Another option might be to implement support for the {{Ignore}} and {{Optional}} conventions from our translation community (as placed in qqq), and then based on that ignore existence validation for translations.

The option for that could be enabled for default and act across things that would otherwise relate to disallowBlankTranslations and requireCompleteTranslationLanguages, which might be more user-friendly long-term.

Thoughts? (see also #69)