add aliases and canonical form

[doriantaylor]

The shared-mime-info database catalogues type aliases and I propose exposing them via new aliases, alias?, and canonical methods:

m = MimeMagic.new('text/xml').canonical
# => <MimeMagic "application/xml">
m.aliases
# => [<MimeMagic "text/xml">]
m.aliases.first.alias?
# => true

This will make it possible to match a wider range of inputs. Happy to do the patch.

[doriantaylor]

Update: I did the patch.