Make timeout warning context configurable?

`spread` currently supports setting the `warn-timeout`, which dictates how long before `spread` displays a message saying a task is running late, with a small amount of output from the task.

Adjusting this can give users more information depending on the task, but some applications have quite verbose output, and the number of log lines displayed can be limiting.

We could consider adding a `warn-context` or `warn-lines` that makes the number of lines shown in the late warning adjustable.

Discussed briefly with @niemeyer this morning with neither an affirmative "yes definitely" or "no definitely not"! Could be useful in some cases.

Start by locating the warn-timeout handling and the code that formats late-task warning output. Determine how the displayed task lines are selected, then make their count configurable and verify that warn-timeout behavior remains unchanged while the configured context limit is respected.

Written by the indexing model from the issue text.