Support blasphem.wad

[mooreye]

Support loading a IWAD named blasphem.wad to launch Heretic. Blasphemer is a Freedoom equivalent of Heretic: https://github.com/Blasphemer/blasphemer

[mooreye]

Currently dsda-doom crashes if you load just blasphem.wad. You have to either rename it to heretic.wad or load as PWAD after the original heretic.wad. Or you can use -heretic flag explicitly.

[kraflab]

As you mentioned, you can load alternate iwads by switching the engine with -heretic, so the wad is already supported. A separate question is "should dsda-doom use some heuristics to guess what game an iwad is for?", to which the answer is probably yes. I'll add that to my todo list, but I will close this issue since I don't intend to add "special" support for custom iwads.