Support ExUnit filters

:warning: Still working on this one.

- [x] ExUnit.configure
  * Made the assumption that you would never have a tag with a value of `nil`. Let me know if we need to allow it.
- [x] Tags support
  * Do we need any kind of option for running all of the tests if all tests are filtered?
  * Testing this was pretty weird. Any thoughts on a better way to keep track of all passed/skipped tests compared to defining a custom formatter?
- [ ] Mix support

Closes https://github.com/elixir-lang/elixir/issues/1898