Initial specs and slight refactoring for Minitel

@brandur @pedro what do you think?
Yeah, @jkakar has mentioned something similar in the past. ie it would be nice to have a mechanism to more explicitly say something like "I invoke foo and it uses this stub". Since the stub can call a block, it is feasible that the block could set some value you could check I suppose, but it isn't as nicely supported as it could be.