Recognize .gen ROMs in Sega MegaDrive as well.

GoodTools ends up renaming roms to .gen, but they're just regular .bin files.
The users could simply rename them back to .bin, but why not accept them
as is anyway? DGen doesn't complain.
Merge pull request #266 from vitorbaptista/master

Recognize .gen ROMs in Sega MegaDrive as well.