Always use color.
Make `described_class` to return innermost described class.

It used to return the outermost described class. It doesn't
happen very often that there is more than one class being
described in the set of nested example groups (and we
generally recommend against it), but this was surprising
behavior to me.

Fixes #1114.
Refine `described_class` inference further.

In the last commit, I made inner `describe SomeClass`
example groups get `SomeClass` as the described_class.
The logic confusingly allowed a `:described_class` metadata
key in an outer group take precedence, which meant that the
behavior depended on whether or not `described_class` was
referenced from the outer group.

This fixes that, making it work consistently:

- Use `:described_class` is present.
- Otherwise, use the group's `describe` arg
  (unless it's something like a string).
- If neither of those, look in the parent and
  recurse.
Merge pull request #1361 from rspec/fix-nested-described-class

Fix nested described class