Add an Auto-fill function

[shsms]

Is your feature request related to a problem? Please describe.
When making changes, lines sometimes get too long. We need something like emacs' auto-fill mode to fold long lines and join short lines.

Describe the solution you'd like
We need a new method autofill(n) with optional n defaulting to 80. It would operate on a narrowed view if present, or on the entire document. We also need an autofill_region(n) that autofills only from mark to point, if a mark is present.