Custom/managed publishing options.

This is related to issues like #1190 and #1475 and I think we kinda mentioned it a couple of times here and there. The point is that there is not a single-size-fits-all solution and we should provide site builders with the means to customize these options in order to fit their case. #1475 is a meta-issue and it sorta focuses on the UI side of things. This issue here is about the underlying cogs that make things work though and it doesn't quite fit the UX spectrum (so not tagging as such)... So, the idea is to: - allow the default, out-of-the-box publishing options/flags to be renamed - allow adding/removing publishing options