This commit introduces mirror_grid, a small application that mirrors gri...

...ds. It does exactly what the name suggests. Huge thanks to Lars Odsæter who made the tool in the first place, and a huge thanks to Andreas Lauser who ported it to the new parser.

Please be mean to me, I need to learn how to make good PRs :-)
Noticed only now: in C++98, open requires a const char*, so use mirrored_eclipsefilename.c_str() for compatibility. I'll fix this.