Allow profiles to be loaded out of plugins.

Still needs some testing and docs, but what do people think?
I dont think this works; the plugin jar name doesn't necessarily correspond to the name of the "task" it contains.  For example `com.keminglabs/cljx` should map to `cljx/profiles.clj` and `lein-cljsbuild` should map to `cljsbuild/profiles.cljx`.

Also I think it would generate invalid keywords, eg. `(keyword "plugin.com.keminglabs/cljx" "profile-name")`.