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.
We have a macro that we use for errors called ```show_error!```, so it would be better to use that.  It's located in ```common/util.rs```.