Allow using of different compilation commands

Hello,

here's a change to allow users to use different compilation commands with cpputils-cmake.
A user could use this for example like here:

    (eval-after-load "cpputils-mode"
      (defun cppcm-create-compilation-command (target-path)
         (concat "ninja -C " target-path)))

This may be sufficient for issue #19 (it would be for my use case).

Thanks,

-Marko

P.S. I am not yet all that familiar with elisp or pull requests. Please feel free to offer suggestions on either one. :)