shorten: Reduce each in a vector of strings to a given length
Description
This is a simple "chop" at k characters, no fancy truncation at
spaces or such. Optionally, this will make unique the resulting
truncated strings. That way, truncation at character 4 of
"Washington" and "Wash" and "Washingham" will not result in 3
values of "Wash", but rather "Wash", "Wash.1", and "Wash.2"