Plurals support

[Nohus]

Added support for <plurals> strings. Quickly hacked together, seems to work fine, but I am not familiar with the project at all, nor do I know PHP, so the code is likely very bad. Opening this PR in the hope this can be useful to someone, it's good enough for my use case.

Some comments:

• Different languages have different types of plurals, which the tool has to know. I have made this configurable and added configuration for the 3 languages I need. Languages without plurals configuration will still work fine, but will prompt for translations for all possible plurals' types, even though not all are used in a given language.
• For showing the English reference translation, I am using the "one" value for the "one" plural, and "other" value for all other plurals ("zero", "few", "many"...).
• I have not tested if this hasn't broken other types of strings (e.g. <string-array>), as I am not using them. Please test before using/merging.

[erickok]

This looks pretty good to me, though I will try to try it out myself before merging.

[Nohus]

Bump