Preliminary support for application/external-editor

Preliminary support for application/external-editor
allows editing wiki pages or files with an external application, by using
an intermediate helper application (such as the reference one in
extensions/ee) which fetches the data and transmits the changes back to the
server.

This version adds external editor support for uploaded files, as well as for
pages. More operations, such as diffs and merges, should support
application/external-editor in the future.

See extensions/ee/README for some more info on the concept. Documentation
will also be placed on http://meta.wikimedia.org/wiki/Help:External_editors