Add mute mode for SBT plugin

This PR adds a "mute mode" for Gatling, currently only used for (and needed by) the SBT plugin.
Therefore, it is hidden from the usage text and from gatling.conf.

This "mute mode" implementation is quite unclean ATM, I intend to get it right for Galing 2M5, along a larger refactoring of Gatling's main class.