Initialization; Persistent, Lazy Filters

These commits introduce initialization hooks and solve the issue of filters being created on require.

The hooks were originally created to literally create the filter when it `stack-initialize` is called, but a re-implementation of filter access (prototyped by @Bruce-Connor) rendered this unnecessary.
Are these keywords necessary in any of these files but the eventual `stack-exchange.el`? They're difficult to really manage; it would be pure repetition.