Prior to this change, if the user's prompt was wider than the terminal, we
would reduce it to just `> `. With this change, attempt to truncate the
prompt.
For each line of the prompt, calculate its width. If the width exceeds
COLUMNS, prepend ellipsis to that line, and start removing characters
until it fits. Escape sequences are skipped.
Fixes#904
We use sphinx with rst for our documentation, and github supports rst
here, so it seems weird to have markdown just for these.
It also allows us e.g. to include the CHANGELOG in the docs without
requiring another build dependency.
