Add meson support to docopt

[germandiagogomez]

Hello everyone. I am using docopt and I would like to add at least minimal meson support (this is the build system I am using).

I sent PR #78 . Is there anything else that I should do? I just have a Mac OS machine and the file is just minimal: just compiles the library and that's it, because it is the way I am consuming it.

[jaredgrubb]

As soon as that PR is approved I will merge it!