Make bitcoin compatible coins pluggable

[gertjaap]

Without needing recompile, just specify the params in a json file and perhaps PoW / Difficulty adjustment functions in a pluggable module

[delbonis]

I think we'd need to refactor how the config is loaded for this to work nicely, or set up a separate "coindefs" directory that has a bunch of json files with settings that looks roughly like a coinparam. Although it shouldn't be too hard.