Optional blueprint list merging

Implemented an option to merge blueprint lists, as suggested a few weeks ago in the thread at the forums. Merges only blueprint lists, but not blueprints themselves, allowing, for example, multiple weapon mods to be played together without crashing the game.
Disabled by default.

Works fine as far as I've tested, though I expect there's plenty of room for improvement, since I'm not really experienced with Python.
I didn't really know what else to add, if anything; do let me know whether there's something that needs to be changed, or whether such a feature is welcome at all :E

Edit:
D'oh, there's a minor typo in merge_blueprint_lists description; it reads "Defailt" instead of "Default"