bugs @ 2cc19f841df2

Die if there are any newlines inside the task text

The `t` file format is line-based, so newlines break the file format.  We'd have
to escape them or something, but that opens a whole can of worms, and `t` is
supposed to be simple, so we'll just forbid them.

Fixes https://github.com/sjl/t/issues/26