Speed improvement for the "lein help" command (with nicer code)

Since you liked the approach @RyanTM and I came up with in #1402 for speeding up the "lein help" command, here is a better version of that.

This depends on the new functions I added to bultitude in https://github.com/Raynes/bultitude/pull/24 .  I have been testing Leiningen against my own personal copy of bultitude on commit 68491b281a5de760fc44e087fffdca2e03b2ef6b using the "checkouts " feature of Leiningen (which is really handy, by the way).

I am still getting a time of about 4 seconds to run `bin/lein help` and it seems to be working.