2160 string edit

Implemented a string-edit windows (as briefly discussed in COUCHDB-2160): when the cursor is on a line that contains a JSON-String (possibly with encoded quotes / newlines), one can open a textarea containing the unescaped text. This can replace much of what the field-edit view of futon could do - and it should help a lot with editing javascript for design docs or e.g. Angular with couch (when the code is actually contained in documents). Feedback is highly appreciated!
I would rename this to `canEditDocString`