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/vagrant.plugins.bash

@@ -0,0 +1,50 @@
+#!/bin/bash
+_vagrant()
+{
+    cur="${COMP_WORDS[COMP_CWORD]}"
+    prev="${COMP_WORDS[COMP_CWORD-1]}"
+    commands="box destroy halt help init package provision reload resume ssh ssh_config status suspend up version"
+
+    if [ $COMP_CWORD == 1 ]
+    then
+      COMPREPLY=($(compgen -W "${commands}" -- ${cur}))
+      return 0
+    fi
+
+    if [ $COMP_CWORD == 2 ]
+    then
+        case "$prev" in
+            "box")
+              box_commands="add help list remove repackage"
+              COMPREPLY=($(compgen -W "${box_commands}" -- ${cur}))
+              return 0
+            ;;
+            "help")
+              COMPREPLY=($(compgen -W "${commands}" -- ${cur}))
+              return 0
+            ;;
+            *)
+            ;;
+        esac
+    fi
+
+    if [ $COMP_CWORD == 3 ]
+    then
+      action="${COMP_WORDS[COMP_CWORD-2]}"
+      if [ $action == 'box' ]
+      then
+        case "$prev" in
+            "remove"|"repackage")
+              local box_list=$(find $HOME/.vagrant/boxes/* -maxdepth 0 -type d -printf '%f ')
+              COMPREPLY=($(compgen -W "${box_list}" -- ${cur}))
+              return 0
+              ;;
+            *)
+            ;;
+        esac
+      fi
+    fi
+
+}
+complete -F _vagrant vagrant
+