Add option for supporting the interleave package

[cpaulik]

I've implemented support for the interleave package for myself and thought it might be interesting for others.

If you don't want to support this since users can easily change the note taking function then feel free to close.

[tmalsburg]

I didn't know about the interleave package. Looks really useful. There is a branch (note-files, see #40) which includes support for note file templates. I would probably include interleave support after that branch is merged. Unfortunately, I didn't have time to work on it recently.