Simple refactoring -- now there's no need to source each one independently.
This continues the move toward the ability to turn things off with everything turned on by default.
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.