Add XL_PATH to allow alternate config directories

[rankynbass]

XL_PATH can now be used to set an alternate config directory. This is useful for multiboxing, and for people who want their config in ~/.local/share/xlcore instead of ~/.xlcore.

Also updated the README with a few environment variables.

[goaaats]

Could this be XLCORE_PATH? Might make it clear that this only works on xlcore. Or we also implement it on Windows and name it XL_USERDIR or something.

[rankynbass]

Alright, I've removed XL_FIRSTRUN from the readme. I'm hesitant to remove the implimentation because it's still checked in a couple of places, and I don't want to break anything.

I have also fixed the readme to no longer reference -RB, and changed XL_PATH to XL_USERDIR.

[acfrazier]

@reiichi001 Is this able to be merged? I was trying to migrate my multibox setup to Linux this morning, and realized that neither XL_PATH nor XL_USERDIR are respected.