Better support for Gettext

[hauleth]

Currently Gettext supports only Language: header and rest is left as is, which can be problematic, as for example Plural-Forms: is crucial for correctly translating a lot of applications (Unicode have list of different language pluralisation rules).

[simonprev]

This seems like a duplicate of #30 The enhancement should be "simple" to implement: Store gettext pluralization rules in each languages.

[hauleth]

Not exactly it is duplicate as there are more headers that could be supported, like PO-Revision-Date, Last-Translator, Language-Team and others (even custom ones like X-Generator).