use partition instead of regex for splitting todos

Removes the wasteful regex for parsing todo items and replaces them with
a call to str.partition. This should hopefully be faster. At the very
least, it's more readable.
Bug: items with blank text would be added

Blank items are added to the todo list due to whitespace. The line
consists of a newline, so len(line) == 1. Fixed by stripping whitespace
before checking line length
Change 'editor' to 'sensible-editor'