Issue #427: fix edge case bug in plural pseudolocalization

[professorplumb]

Fixes issue #427.
Depends on PR #419 - the test added here will fail on this branch until that PR is incorporated.

This fix will still fail in the case that a variable placeholder in a template string inside a plural macro is followed by a single word which matches one of the plural rules, e.g. {count, plural, one {{countString} other} other {{countString} others}}.

This is a much more extreme edge case, though - it must be

1. A template string
2. Inside a plural expression
3. With a variable placeholder
4. Followed by zero|one|two|few|many|other
5. With no other words or punctuation after that word
6. With pseudolocalization enabled

IMO this is an acceptable tradeoff for a more readable regular expression.

[tricoder42]

Thank you, @professorplumb. I think this is the best what we can get with regular expressions. 👍

Other method is use message-parser tokenizer, traverse through text tokens, pseudolocalize them and then join them back to message format, but that seems to be overengineered at the moment.

This patch looks good, thanks!

[tricoder42]

Released in 2.7.3