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/base.plugin.bash

@@ -0,0 +1,113 @@
+#!/bin/bash
+
+# For generic functions.
+
+function ips {
+  ifconfig | grep "inet " | awk '{ print $2 }'
+}
+
+function down4me() {
+  curl -s "http://www.downforeveryoneorjustme.com/$1" | sed '/just you/!d;s/<[^>]*>//g'
+}
+
+function myip {
+  res=$(curl -s checkip.dyndns.org | grep -Eo '[0-9\.]+')
+  echo "Your public IP is: ${bold_green} $res ${normal}"
+}
+
+# Make a directory and immediately 'cd' into it
+
+function mkcd() {
+  mkdir -p "$*"
+  cd "$*"
+}
+
+# Search through directory contents with grep
+
+function lsgrep(){
+  ls | grep "$*"
+}
+
+# View man documentation in Preview
+pman () {
+   man -t "${1}" | open -f -a $PREVIEW
+}
+
+
+pcurl() {
+  curl "${1}" | open -f -a $PREVIEW
+}
+
+pri() {
+  ri -T "${1}" | open -f -a $PREVIEW
+}
+
+quiet() {
+	$* &> /dev/null &
+}
+
+banish-cookies() {
+	rm -r ~/.macromedia ~/.adobe
+	ln -s /dev/null ~/.adobe
+	ln -s /dev/null ~/.macromedia
+}
+
+# disk usage per directory
+# in Mac OS X and Linux
+usage ()
+{
+    if [ $(uname) = "Darwin" ]; then
+        if [ -n $1 ]; then
+            du -hd $1
+        else
+            du -hd 1
+        fi
+
+    elif [ $(uname) = "Linux" ]; then
+        if [ -n $1 ]; then
+            du -h --max-depth=1 $1
+        else
+            du -h --max-depth=1
+        fi
+    fi
+}
+
+# One thing todo
+function t() {
+	 if [[ "$*" == "" ]] ; then
+		 cat ~/.t
+	 else
+		 echo "$*" > ~/.t
+	 fi
+}
+
+# Checks for existence of a command
+command_exists () {
+    type "$1" &> /dev/null ;
+}
+
+# List all plugins and functions defined by bash-it
+function plugins-help() {
+    
+    echo "bash-it Plugins Help-Message"
+    echo 
+
+    set | grep "()" \
+    | sed -e "/^_/d" | grep -v "BASH_ARGC=()" \
+    | sed -e "/^\s/d" | grep -v "BASH_LINENO=()" \
+    | grep -v "BASH_ARGV=()" \
+    | grep -v "BASH_SOURCE=()" \
+    | grep -v "DIRSTACK=()" \
+    | grep -v "GROUPS=()" \
+    | grep -v "BASH_CMDS=()" \
+    | grep -v "BASH_ALIASES=()" \
+    | grep -v "COMPREPLY=()" | sed -e "s/()//"
+}
+
+# back up file with timestamp
+# useful for administrators and configs
+buf () {
+    filename=$1
+    filetime=$(date +%Y%m%d_%H%M%S)
+    cp ${filename} ${filename}_${filetime}
+}