Canonical types and aliases

[jeremy]

• MIME type aliases are now supported.
• Aliases are resolved to their canonical type in all APIs.
• Introduce MimeType.canonicalize type, instead_of: old to override a Tika canonical type with our own, essentially renaming the type and making the old type an alias of the new one. Common scenario with types like WAV with multiple competing historical types, RFCs that aren't actually followed, and browser support trumping them all. This allows us to give preference to browsers' actual MIME type support while keeping Tika's file extensions and magic byte matchers.
• Warns when extending a type with preexisting extensions, parents, etc. and when extending an aliased type.