Lookup mechanism:

1. Check whether an editor was given with the -E flag:

     topydo edit -E vim

2. Use the value of $TOPYDO_EDITOR
3. Use the value in the configuration file:

   [edit]
   editor = vim

4. Use the value in $EDITOR
5. Use 'vi' if the above fails.

This makes it easier to invoke an editor as vim with additional
parameters, for instance by adding the full todo.txt file to the
completion options (see the tip in issue #164). This trick was added to
the configuration file as a comment.