Make it easier to enable lein-shorthand for other tasks

[gfredericks]

My use case is wanting to run lein ring server-headless, which has an optional feature of starting an nrepl server, and have the shorthand stuff enabled for when I use that server.

I'm thinking the cleanest way to do this would be to rewrite the implementation to be via a :lein-shorthand profile that perhaps gets enabled via a hook for the repl task, but that then users can manually add to other tasks.