Make all aliases optional (a la plugins)

This continues a pattern that was introduced in ffa45b0 and refined
further in later commits.  This enables all aliases by default, but can
be disabled by any user if they remove the appropriate
aliases/enabled/*.bash file.

aliases/available/osx.aliases.bash

@@ -0,0 +1,20 @@
+#!/bin/bash
+
+# Desktop Programs
+alias fireworks="open -a '/Applications/Adobe Fireworks CS3/Adobe Fireworks CS3.app'"
+alias photoshop="open -a '/Applications/Adobe Photoshop CS3/Adobe Photoshop.app'"
+alias preview="open -a '$PREVIEW'"
+alias xcode="open -a '/Developer/Applications/Xcode.app'"
+alias filemerge="open -a '/Developer/Applications/Utilities/FileMerge.app'"
+alias safari="open -a safari"
+alias firefox="open -a firefox"
+alias chrome="open -a google\ chrome"
+alias chromium="open -a chromium"
+alias dashcode="open -a dashcode"
+alias f='open -a Finder '
+alias textedit='open -a TextEdit'
+alias hex='open -a "Hex Fiend"'
+
+if [ -s /usr/bin/firefox ] ; then
+  unalias firefox
+fi