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.

bash_it.sh

@@ -37,7 +37,12 @@ do
 done
 
 # Aliases
-FUNCTIONS="${BASH}/aliases/*.bash"
+if [ ! -d "${BASH}/aliases/enabled" ]
+then
+  mkdir "${BASH}/aliases/enabled"
+  ln -s ${BASH}/aliases/available/* "${BASH}/aliases/enabled"
+fi
+FUNCTIONS="${BASH}/aliases/enabled/*.bash"
 for config_file in $FUNCTIONS
 do
   source $config_file