Implemented "read from stdin" in the 'eel' executive

Implemented the -s switch, and some supporting logic to make it work.

The main<> entry point is optional when reading from stdin (no error
message), but do note that that means the script code ends up in the
root scope of a module, which means it runs at module initialization
time, before any subsystems (like SDL...) have been initialized!

Renamed eel_load_from_mem() as eel_load_buffer().