Installer support

This was submitted by @josh-guest as a comment on my blog entry:

This looks great! Would it make sense to provide a low-priority (expert) debian-installer question for preseeding, for names of extrepo repositories to enable? That could then get processed before the questions to install additional packages. I'd find that incredibly useful, and would use it in every installation I do.

While that seems like a good idea and should be pretty easy to implement, I'm currently more focused on working out the kinks in the current system, and making sure that this is a good idea at any rate, before moving on to adding that kind of stuff.