Add (ported) nmaster-ncol.c

[DanielFabian]

This is a port from the DWM website. I use a 55" 4k screen and nmaster is really useful in that scenario.

[prabirshrestha]

Doesn't seem to compile with variable being undefined. It would be good to include the file in "config.h.in" with rest of the algorithms.

[prabirshrestha]

src/nmaster-ncol.c:12:17: error: use of undeclared identifier 'nmaster'; did you mean
      'nmasters'?
                nmasters[i] = nmaster;
                              ^~~~~~~
                              nmasters

[prabirshrestha]

Might be worth adding comments in README or above the file in nmaster-ncol.c about section on dealing with large monitors since this does require a bit more config beside just setting up the layout.

[goyalyashpal]

@DanielFabian @prabirshrestha ping