The EELBox bindings are now a build time option

EELBox can now be disabled by passing -DBUILD_EELBOX=NO to CMake.

Binding module registration and SDL subsystem init has been moved
into eelbox.[ch] in the eelbox directory, keeping SDL details and
other noies out of the 'eel' executive.

Merged -o and -s options back into the executive. (Those were only
in the non-EELBox one before - no that they're really implemented
or anything anyway...)