External formatter module support, fixed & checked-in rules and formatters

As proposed in https://github.com/palantir/tslint/issues/9#issuecomment-25822098 and https://github.com/palantir/tslint/issues/9#issuecomment-25852476.

I left it in 3 commits so each of the 3 individual Diff Stats are sort of usuable.