Added some make targets.
Updated/added the following:
 * converted run-in-* functions to accept functions as parameters and display
   the output from those appropriately (while processing current dir, or while
   processing other dirs, depending upon the caller)
 * created new demo-run-in-* functions that are actual plugin commands
 * created a new command (and supporting functions) for listing the plugins
   configured for the current directory
 * created a new command (and supporting functions) for displaying the plugin
   command help when displaying the general rebar command help
Updated README.