Introduce shoes-highlighter gem

Although we might eventually want to phase it out entirely, it's desirable to get Shoes' highlight code 1) included and 2) separated into its own gem. (See #257and #934 for context).

This introduces the gem in the newly proposed structure (#931), where we have a `/shoes-highlighter` directory at the root and everything within that laid out like a typical gem. So this also proves to be a fun experiment in project structure before we try to move more pieces around.

Things I've found:
* There seems to be a problem with the pathing right now that's fouling up bundling on Travis with this branch. Will have to get that fixed.
* We've previously relid on the overlapping structure of the gems in the project to avoid having to lean actually run with Bundler enabled. I found that I couldn't just `bin/shoes` anymore after the new gem came in with it's own separate directory, but had to `bundle exec bin/shoes`. With [this commit](0991a111319144bb1fecc1b6efc3ffca49d31d64) I got rid of the requirement by implicitly pulling Bundler in when I find a `Gemfile` in the current directory. Hope that's acceptable.
* Once we get all the pieces moved, there will be some additional work on all the Rake tasks, but I want to see them actually moved and just change it then.

Assuming this all gets buyoff, I'll probably go to moving the gems one at a time, with `shoes-swt` next I think. Or maybe that will prove to be painful and I'll just bite doing the bigger move... guess won't know till I get there :smirk: 
That's because the `shoes-highlighter.gemspec` is under the `shoes-highlighter` subdirectory, not in the root like the other gemspecs. As we pull things into more independent gem directories, that'll be how we'll need to reference them. (Suspect we could also do a `gem 'shoes-higlighter', path: 'shoes-highlighter'` if that's clearer, but haven't tried...)