add parents and lineage methods

[doriantaylor]

I have a subclass of MimeMagic that I use that has a handful of useful methods:

• .binary? will sample an input to determine if it's binary (or text)
• .default_type will return application/octet-stream if the input is binary, text/plain otherwise
• #parents will return the list of a type's immediate parents
• #lineage will return a type's full list of ancestors

I also make a cursory affordance for type parameters in the constructor and inspect methods.

I'm filing this issue rather than coming in hot with a PR because I also change the behaviour of .by_magic and .all_by_magic, such that they will always return a value (the value of .default_type); I figured that merited some discussion. I'm happy to do the patch that addresses this and another issue.

[doriantaylor]

Update: I did the patch.