add aliases and canonical form

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