Plugins and stuff

[Southclaws]

While playing with #2 I realised I'm going to have to write some kind of generic way to declare:

1. things that contribute to props structs (via tags) at build-time
2. things that hydrate props at request-time

I'm also going to attempt to name these two concepts to make discussion, implementation and documentation easier!

1. shall be called "Providers", this will probably come with a Provider interface at some point.
2. shall be called "Extractors" because they extract information from requests!

A provider will be registered with the library (probably some singleton or something) and that provider will inform Pocket of a struct tag and a function to call when that struct tag is encountered in props. That function may return arbitrary metadata related to that field which can be recalled at request-time.

A provider must also supply an extractor which will get access to the incoming request and have the ability to write to the props reflect.Value field that it owns.

After this, I may implement the existing features as "plugins" to test the thing out.