Add :verbose option to HTML::Proofer to give progress as it goes

This gives users insight into the progress of HTML Proofer as well as mitigates [a Travis timeout due to lack of output](https://travis-ci.org/benbalter/benbalter.github.com/builds/25637878).

- [x] Add `--verbose` to CLI
- [x] Add `:verbose` to HTML Proofer, defaults to `false`
- [x] Add `debug` output for checks (using [Yell](https://github.com/rudionrails/yell))
- [x] Upgrade to HTML Pipeline 1.8 and escape_utils 1.0 (to fix Ruby 2.1 problem)