Create the concept of enabled plugins

This allows users to disable a plugin without completely removing it.
Instead, they simply remove the `plugins/enabled/*.bash` file for the
plugin they want to disable.  This continues the concept of "everything
on" while providing greater flexibility to future users.

It might be a good idea to allow turning these off by default in the
future and allowing not only the `plugins/enabled/*.bash` files but also
an array of `<plugin_name>` values that would search for
`plugins/available/<plugin_name>.plugin.bash` to enable them.  That
method would make it easier for people custom tune their plugins from
within their `.bash_profile` script.

plugins/available/virtualenv.plugin.bash

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+# make sure virtualenvwrapper is enabled if availalbe
+[[ `which virtualenvwrapper.sh` ]] && . virtualenvwrapper.sh
+