remove Hash inheritance in exclusion & inclusion filters
Rename `include!` and `exclude!`.

I think `include_only` and `exclude_only` are
more clear about what the methods do.
Remove unneeded methods.

Having a base class implementation that raises
`NotImplementedError` can be useful for classes
that users are meant to subclass, but this is
internal and it provides little value.
Remove `method_missing`.

We're trying to shrink/tighten the interface here;
no reason to keep supporting the full hash interface.
Add changelog entry.
Merge pull request #1371 from rspec/pr-1248-merge-prep

Pr 1248 merge prep