Add support for Echo effect

[ericoporto]

OpenAL Soft has an implementation of the Echo effect under EAX, it would be nice if it was supported in mojoAL.

Using echo could be useful for real-time effects in games like simulating a cave area.

There are some interesting reverb presets in this project here: https://github.com/ProjectDreamland/area51/blob/c804e7f3a3a618756cbb7eb237c746227f10f16b/xCore/3rdParty/Miles6/src/M3d/Ds3dbase/Eax.h#L41