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/hg.plugins.bash

@@ -0,0 +1,14 @@
+#!/bin/bash
+hg_dirty() {
+    hg status --no-color 2> /dev/null \
+    | awk '$1 == "?" { print "?" } $1 != "?" { print "!" }' \
+    | sort | uniq | head -c1
+}
+
+hg_in_repo() {
+    [[ `hg branch 2> /dev/null` ]] && echo 'on '
+}
+
+hg_branch() {
+    hg branch 2> /dev/null
+}