Implement fields cutting

Adds an implementation for cut_fields() and creates a separate funtion
for the --output-delimiter, for performance reasons.

This implementation relies on ::read_until() to find the newline for us
but read_until() allocates a vector every time to return it's result.
This is not ideal and should be improved upon by passing a buffer to
read().

This follows/implements the POSIX specification and all the GNU
conventions. It is a drop-in replacement for GNU cut.

One improvement to GNU is that the --delimter option takes a character
as UTF8 as apposed to single byte only for GNU.

Performance is about two times slower than that of GNU cut.
Remove ranges' sentinel value

All cut functions iterate over the ranges and therefore it only adds an
extra iteration instead of improving performance.
Add try --help message to errors