Add ability to get metadata

[rmccue]

Hi there! Thanks for the great library :)

I'm working on a project right now that involves replacing an existing gettext implementation, and in order to do so, I need to be able to effectively export parts of the .mo data; namely, I need to be able to pull the Plural-Forms metadata back out verbatim.

parse_catalog calls out to parse_metadata(), but the map value is discarded once the charset and plural forms are read out; rather than calling parse_metadata() manually again later, I think it would make sense to store the map in the Catalog struct?

(I can PR this if you're interested!)