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/todo.plugin.bash

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+export TODO_DIR=$HOME/.bash_it/custom  # store todo items in user's custom dir, ignored by git
+export TODOTXT_DEFAULT_ACTION=ls       # typing 't' by itself will list current todos
+
+export TODO_SRC_DIR=$HOME/.bash_it/plugins/available/todo
+
+# respect ENV var set in .bash_profile, default is 't'
+alias $TODO='$TODO_SRC_DIR/todo.sh -d $TODO_SRC_DIR/todo.cfg'
+
+export PATH=$PATH:$TODO_SRC_DIR
+source $TODO_SRC_DIR/todo_completion   # bash completion for todo.sh
+complete -F _todo $TODO                # enable completion for 't' alias