Support KCR fileformat

[ColinPitrat]

From #38 (comment)

Would it be possible to add support for the .kcr format?

https://github.com/redbug26/crocods-core/wiki/kcr

From the info supplied:

Recently, I build a cabinet arcade and I want to play Amstrad games on it. The existing file format are not enough.

I want:

Quick start:
I don’t need the splash screen
I don’t want to type the RUN" things
To play with a joystick. Some game need keyboard to select the Joystick (eg., on Boulderdash, you need to type the Enter key to begin to play)
What I propose is the .kcr file format.

.kcr is a zipped archive that contains 4 files.

capture.gif: a screenshot from the game (used for a splash screen for instance)
disk.dsk: the main disk
snapshot.sna: the state from where the game will begins
settings.ini: some key mapping.

And here you can download a couple of examples.

[ColinPitrat]

Because we already support loading from zip, this should be relatively easy:
Read .kcr files like .zip file but look only for the .sna file in it (it contains all we need, the disk shouldn't be necessary - we could load it too still just in case).