Don't run `before :all` blocks when all the examples are disabled

RSpec will already avoid calling `before :all` when all of the
examples are disabled, but only when it knows that they have been
filtered. We can explictly filter to share this functionality.

Closes #12
Merge pull request #18 from shepmaster/dont_run_unused_before

Don't run `before :all` blocks when all the examples are disabled