Identically to what we've added to the module generators, let's raise a
coding exception if generating a block or a repository triggers the
theme and output initialisation.
* Refactor the block generator base class, to remove the amount
of duplication required in base classes.
* Change the defaults that are filled in to be a little more natural.
* Make the Behat step 'Given the following "block" exist:' work.
David Mudrák