Allow templater UI to ask for arbitrary fields

[mitcho]

Currently the templater asks for the desired number of lists and the "code", which is used to replace {{code}}. However, some skeletons may have no {{code}} tag, and some skeletons might have a use for some other arbitrary tags.

So, the idea is to have the templater look ahead at the skeleton for unknown fields, and ask for them in the UI.

There should also be a corresponding command line interface, maybe of the form --code=TEST or --code TEST.