Allowing dashes in filenames

This, adds a new meta character `&` which denotes a file name including dashes. This fixes #150.

Please note, that there might be other commands which should use the new meta character instead of `%`, if that value can't be a variable.

**DISCLAIMER**: As my computer is not set up for development and testing C#, I'm not sure that this code works as promised. (I really have to set up my computer for developing KSP plugins)