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/ssh.plugins.bash

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+function add_ssh() {
+  echo -en "\n\nHost $1\n  HostName $2\n  User $3\n  ServerAliveInterval 30\n  ServerAliveCountMax 120" >> ~/.ssh/config
+}
+
+function sshlist() {
+  awk '$1 ~ /Host$/ { print $2 }' ~/.ssh/config
+}