Add a minimal custom editor

[paul90]

Adds a custom editor that adds a language input field. This will be used for highlighting.

[WardCunningham]

This is so much better than I was thinking I would do to set that field with Mech,
(There is something fishy with detecting loss of focus as we noticed together.)

[WardCunningham]

Tip: that focus out commit looked real complex until I added ?w=1 to the diff url to ignore white space changes.