`v-editable-text` directive, modeled after `v-model`

Draft directive for contenteditable. Includes unit
tests for the contenteditable-selection utility, but they're
not currently passing. Also needs unit tests/integration tests
for the directive itself.

@jordangarcia Will you take a quick look at this before I open a PR to the main Vue repo? It's almost exactly the same as code you reviewed previously, but since you've committed to the main Vue repo before it'd be great to get some input from you. I'm having trouble with the unit tests at the moment, but I don't really have time to dig into the environment to debug what's going wrong.