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/rvm.plugin.bash

@@ -0,0 +1,16 @@
+#!/bin/bash
+switch () {
+  rvm $1
+  local v=$(rvm_version)
+  rvm wrapper $1 textmate
+  echo "Switch to Ruby version: "$v
+}
+
+rvm_default () {
+  rvm --default $1
+  rvm wrapper $1 textmate
+}
+
+function rvm_version () {
+  ruby --version
+}