Register todo.txt as a file type instead of looking for exact filenames.

[nycki93]

Some users like myself have additional "todo" formatted lists, like maybe.txt and projects.txt. It would be nice for these to get the benefits of the todo.txt formatting and commands. It would also be nice if I could assign user snippets on a per-file-type basis, for instance to recognize that "today" completes to "2018-30-09" (or appropriate) when editing a todo-formatted file. Currently, todo.txt registers as "plain text" even when the custom syntax highlighting is on, so I can't do these per-file-type settings.