Add profiles to plugin

An example of profiles that could be added given the pending :downstream
profile in leiningen.

An example of what a project.clj looks like when using cljx and
cljsbuild plugin profiles is at
https://github.com/hugoduncan/downstream-profile-examples/blob/master/cljx-cljsbuild-example/project.clj

This is a pull request for discussion only - it relies on
https://github.com/technomancy/leiningen/pull/1673 which has yet to be
merged (and still contains bugs).