Compress to CHD support

[msauder]

With emulators on iOS, this would be hugely beneficial. After reading MAME documentation I'm not sure how easy it would be to implement.

[gingerbeardman]

Oh this is a fun idea.

I often do CHD using chdman (binary is part of MAME download).
It would require Keka to accept valid BIN/CUE pairs.

We could also add compressed iso for PSP using ciso but that is much more niche.