Function to return MIME Type list

[kelson42]

We need functions to return the list of MIME types listed in the header region of the file.

• Iterators that loop on the list of MIME types in the ZIM file.
• An MIME type class, with functions to return field name, type, size, descriptions, etc.

Moved from https://phabricator.wikimedia.org/T51062

[kelson42]

See also https://github.com/openzim/libzim/issues/73