[V5 BETA1] ZX 81 rom detection

Hello peepz

(this is for Beta 5)

Just a small note. The ZX 81 did not recognize my roms. I checked the ES_Systems.cfg and for the extensions it does not have ".p" in the list. Just adding .p was enough to fix it. I know you can zip the .p files and it will work but I like having it raw and other systems do read both zipped and unzipped file extensions so figured I would mention this.

Hello
.P extension should work only with 81-libretro core.
BTW, the change have been proposed.