src/lang.c: Add Mercury language (*.m, *.moo)

Mercury language (http://www.mercurylang.org)
tests/list_file_types.t: Add test for Mercury file types
tests/list_file_types.t: add missing whitespace
Merge pull request #557 from sebgod/language_mercury

src/lang.c: Add Mercury language (*.m, *.moo)