Adding es_systems config macro for parent directories

[eib]

I'm trying to make my EmulationStation configuration "portable", as in self-contained to a single folder (anywhere) and that means getting es_system.cfg to use relative paths for the system <command> values. My target is Windows PC (using builds from jdrassa's mirror of this repo), which makes my task harder than simply using bash magic like $(dirname %ROM%)/....

In digging around this codebase and the docs, I see that there are already 3 macros (%ROM%, %BASENAME%, and %ROM_RAW%) that work with <command>. There's also some tilde magic, but that only applies to <path>.

I'd like to see an additional "macro" or two: the directory of the executable (could be called %ES_HOME%) and the ROM's parent directory (could be called %ROM_DIR%). I think that would give me (and others) enough flexibility to define the "perfect" folder structure on our own.