add todo.txt-cli as a plugin

Since it is referenced in the default .bash_profile and in aliases and
themes, why not make it an official plugin?

This creates a 'todo/' directory in /plugins/available, and installs
todo.sh and friends there.  Tab completion and the 't' alias should work
out of the box.

Note: it was also necessary to modify .gitignore to ignore the files
todo.sh generates in custom/.

plugins/available/base.plugin.bash

@@ -137,18 +137,6 @@ usage ()
     fi
 }
 
-t ()
-{
-    about one thing todo
-    param if not set, display todo item
-    param 1: todo text
-	if [[ "$*" == "" ]] ; then
-	    cat ~/.t
-	else
-	    echo "$*" > ~/.t
-	fi
-}
-
 command_exists ()
 {
     about checks for existence of a command