Keyboard trap upon tabbing into editor

[marcysutton]

Keyboard-only users get stuck in a keyboard trap when tabbing into the editor. It makes it impossible to exit, a huge blocker for people who can't use a mouse or trackpad. One quick and dirty solution would be to wire up the escape key once you tab into the text editor...maybe it could send focus to the next tab stop, allowing you to keep going through the page?

In an ideal world, editing with the keyboard would only start once you'd pressed an "enable editor" button, where combined with the escape key to exit, the user would thus be making a more conscious choice to toggle edit mode. But that would be a way bigger change, where an escape key + focus management might do the trick in the meantime. I'll try to put a PR together since I have to fix this for our site.

Edit: this must have started occurring in the latest version, because I'm not experiencing a keyboard trap with an older one.

[marcysutton]

I decided to add an option to override the tab key behavior instead, which was a pretty easy solution leveraging the existing code. See PR #82.