Version 5.0.0pre2

I know, I know, it's actually been two days :<
Anyway, here's the changelog:

Added external localization directory support: ${lang.dir}.
Merged `clean` target with the to-be-deleted files' respective targets
(will improve that).
Updated Forge to 6.6.0.518.
Re-added common.in.dir.
Now checks for MCP compile failure (thanks nallar!).