implemented "nl"

A working version of "nl" that comes with a few basic tests and should run reasonably fast.
Todo: More tests (multiple files, weird combinations of parameters, ...) and a better help (then again, the original's isn't much better).
Closes #231.
You should keep the braces consistent.  The brace here is different from that at line [213](https://github.com/uutils/coreutils/pull/238/files#diff-76d282eccdcb924d4375b4738641a9d0R213).