Allow specifying a fan index in M106 (and M107)

[Wallacoloo]

Although undocumented on the Reprap gcode wiki, Teacup (and I suspect other firmwares) supports an optional P parameter in the M106 command to specify a specific fan to affect.

Although Teacup defaults to toggling fan 0 when no P value is given, I think it would be more appropriate to have the command affect all fans when no P value is present (as a lack of a P value implies that the host doesn't realize there are multiple fans, so it wouldn't know to configure the other fans too). However, it might be best to have it default to P=0 for compatibility or ease of implementation.