feature: add --executable flag

[IvarWithoutBones]

I recently found out that this program does not work well in combination with the seamless co-op mod, because its launched through a executable different from eldenring.exe or launch_game_protected.exe. I resolved this problem by adding an --executable flag which allows you to override the path to the executable, relative to the games folder.

I've also fixed some linter warnings, and added an example to the readme that explains how to use this new flag with the co-op mod, since its fairly popular.

Tested with the following:

• With --executable.
• With --with-eac.
• Neither --executable nor --with-eac.
• Both --executable and --with-eac, this throws an error as these flags are mutually exclusive.

[gurrgur]

Thanks for this. Looks fine so I'll merge :)