Add the ability to direct the execution towards one branch

[palinatolmach]

As identified by @lisandrasilva, the only way to force Kontrol to execute a single branch is to refute other branches. It, however, might be inconvenient in case there're many branches (e.g., 60 as in a recent engagement). We should add a separate option that would allow kontrol prove to only explore a single branch within a run.