Add option to use a specific generator

[rafaelsamenezes]

Is your feature request related to a problem? Please describe.
I'd like to run only KLEE or only libFuzzer (or any future generator)

Describe the solution you'd like
Add the --generator option with klee and libfuzerr as valid values

Describe alternatives you've considered
None

Additional context
It should be simple, the major change will be at the wrapper of the interaction between klee and libFuzzer will have to be refactored.