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/.

.gitignore

@@ -1,6 +1,6 @@
 */enabled/*
 .DS_Store
-custom/*.bash
+custom/*
 !custom/example.bash
 .rvmrc
 aliases/custom.aliases.bash