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/osx.plugin.bash

@@ -0,0 +1,37 @@
+#!/bin/bash
+
+function tab() {
+  osascript 2>/dev/null <<EOF
+    tell application "System Events"
+      tell process "Terminal" to keystroke "t" using command down
+    end
+    tell application "Terminal"
+      activate
+      do script with command "cd \"$PWD\"; $*" in window 1
+    end tell
+EOF
+}
+
+# this one switches your os x dock between 2d and 3d
+# thanks to savier.zwetschge.org
+function dock-switch() {
+
+    if [ $(uname) = "Darwin" ]; then
+        
+        if [ $1 = 3d ] ; then
+            defaults write com.apple.dock no-glass -boolean NO
+            killall Dock
+
+        elif [ $1 = 2d ] ; then
+            defaults write com.apple.dock no-glass -boolean YES
+            killall Dock
+
+        else
+            echo "usage:"
+            echo "dock-switch 2d" 
+            echo "dock-switch 3d."
+        fi
+    else
+        echo "sorry. you're currently not using os x"
+    fi
+}