Combokeys

See this PR for how this is used in tests.

We would have to run our tests in OS X, at least once, to see that this is sane.

Or, at the very least, simply manually test the keyboard shortcuts in OS X in the editor.

I don't have anything with Mac OS on it and reading around the interwebs lets me know that installing it in VirtualBox is hackish by definition.

@winhamwr, do you have access to a Mac?

I would test the following:

1. Undo/redo works with `command+z` and `shift+command+z`, respectively.
1. Undo/redo do not trigger with `ctrl+z` and `ctrl+y`, respectively.
1. Bold and Italic work with `ctrl+b` and `ctrl+i`, respectively.
1. The above in Chrome, Firefox and Safari.
1. Run the test suite, if it isn't too much of a hassle, in Chrome, Firefox and Safari. I expect Safari to fail in a surprising amount of tests, because the closest things to it that we test is PhantomJS.