Allow selecting mutators from CLI

[devdanzin]

It should be possible to select which mutators will be used in a fuzzing run instead of having them being hardcoded. We could use the same solution fusil uses for modules: a default of ˋ*ˋ means "use all mutators", a comma-separated list of mutator names selects those present in the list.

Since, unlike modules, the list of possible mutators is small-ish, we can ao add an option to print all of their names with short descriptions.

In ASTMutator, instead of a list of mutators now we'll need to have a dictionary mapping the names to the mutators.