Make truncating configurable

[Jelleas]

Hi there! Over the years we've had a few text-processing assignments here in Amsterdam that are quite verbose. That meant that the default CLI output of check50 is always truncated, and useless most of the time. The web-view solves this, but isn't always available in my setting.

Some time ago I've hacked (breaking all encapsulation) this together to make truncating configurable per assignment: https://github.com/minprog/checks/blob/eadefd4b739d580fbfe4105d150f07d07d0cbe8a/helpers/helpers.py#L8

Currently the truncating length? is burrowed in the method _raw in _api.py and hardcoded on the magic number 15:

check50/check50/_api.py

Line 509 in ab2aa99

if len(s) > 15:

Maybe in 4.0.0, could we have something like internal.set_truncation() or api.set_truncation()?