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/dirs.plugins.bash

@@ -0,0 +1,52 @@
+#!/bin/bash
+
+# Directory stack navigation:
+#
+# Add to stack with: pu /path/to/directory
+# Delete current dir from stack with: po
+# Show stack with: d
+# Jump to location by number.
+
+# Show directory stack
+alias d="dirs -v -l"
+
+# Change to location in stack bu number
+alias 1="pushd"
+alias 2="pushd +2"
+alias 3="pushd +3"
+alias 4="pushd +4"
+alias 5="pushd +5"
+alias 6="pushd +6"
+alias 7="pushd +7"
+alias 8="pushd +8"
+alias 9="pushd +9"
+
+# Clone this location
+alias pc="pushd \`pwd\`"
+
+# Push new location
+alias pu="pushd"
+
+# Pop current location
+alias po="popd"
+
+function dirs-help() {
+  echo "Directory Navigation Alias Usage"
+  echo
+  echo "Use the power of directory stacking to move"
+  echo "between several locations with ease."
+  echo
+  echo "d	: Show directory stack."
+  echo "po	: Remove current location from stack."
+  echo "pc	: Adds current location to stack."
+  echo "pu <dir>: Adds given location to stack."
+  echo "1	: Chance to stack location 1."
+  echo "2	: Chance to stack location 2."
+  echo "3	: Chance to stack location 3."
+  echo "4	: Chance to stack location 4."
+  echo "5	: Chance to stack location 5."
+  echo "6	: Chance to stack location 6."
+  echo "7	: Chance to stack location 7."
+  echo "8	: Chance to stack location 8."
+  echo "9	: Chance to stack location 9."
+}