Add Clingo threading support

[sahil1105]

Add an option to run clingo operations using multiple threads using the -t flag.
e.g.

clingo -n0 foo.pl -t8

would run clingo on foo.pl with 8 threads, potentially speeding up the computation significantly.
Would go in the run_clingo(..) function most likely. Will also need to add this flag to the NB-extension.