Memoize user/profiles.
Add support for ~/.lein/profiles.d. Closes #871.

Add support for profiles.d within the user's Leiningen home directory. All .clj
files within LEIN_HOME/profiles.d will be treated as maps merged into the
original profiles.clj (if existing).

Erroneous .clj file inside profiles.d emits an error message and will be treated
as nil.

A profile defined multiple times will error and "crash" the loading of profiles,
such that profiles return nil.

The errors will only be emitted once for user-friendliness.
Document profiles.d functionality.