Use stdin when no files are specified
Remove 'wc' from error message in open()
Do not combine adjacent ranges
Improve performance of cut_bytes()

Still an order of magnitude slower than GNU cut.
Add cut_characters implementation, based on cut_bytes

This implementation uses rust's concept of characters and fails if the
input isn't valid utf-8. GNU cut implements '--characters' as an alias
for '--bytes' and thus has different semantics, for this option, from
this implemtation.