Added support for build args

Hi,

I made a few small changes to the module to enable build args to be passed to the constructor of each plugin. Basically I just allowed either an ArrayRef (old behavior, backwards compatible) or a HashRef (new behavior) to be passed to plugins() and adjusted a couple of internals accordingly.
One thing I did notice is that the MooseX::Role::Pluggable::Plugin role adds a name and parent attribute to each plugin class. I was considering that maybe changing those to something a little less common might be helpful, perhaps mxrp_name, mxrp_parent. Maybe I can find some time to hack that in if you would be interested in pulling.