Deal with multilingual ZIM

- if one specified language: filename=gutenberg_lang_all_date.zim and ZIM title mentions the language
- between 2 and 5 specified languages: filename=gutenberg_all_date.zim and ZIM title mentions the languages
- more the 6 specified languages: filename=gutenberg_all_date.zim and ZIM title mentions no language indication
- no specified languages (=all languages): filename=gutenberg_mul_all_date.zim and ZIM title mentions the 'mul' language

In any case all the specified languages are written in the 'languages' metadata, comma-separated.
Merge branch 'master' of github.com:kiwix/gutenberg

Conflicts:
	gutenberg/zim.py
Merge branch 'master' of github.com:kiwix/gutenberg