Add support for OpenRaster file format

[chrsmth]

OpenRaster is just another zip wrapped format with unencrypted mimetype file containing image/openraster. I believe searching for mimetypeimage/openraster at offset 30 is the correct way to detect it.

Tested with ora files generated by GIMP & Krita.

spec reference:
https://www.openraster.org/baseline/file-layout-spec.html

[coveralls]

Coverage remained the same at 96.407% when pulling 9ec92ee on chrsmth:support-ora into 71f0e2b on gabriel-vasile:master.

[gabriel-vasile]

image/openraster cannot be used, unless it's registered at IANA. I opened an issue here.