Adding PCFG to slow_candidates

[lakiw]

Oh, and the ruleset needs to be generated by the Python version of the PCFG toolset, (available here: https://github.com/lakiw/pcfg_cracker)

I currently have no plans of porting the training program over to C, since it heavily relies on several Python libraries. Once a ruleset is generated and saved to disk though, either the Python PCFG or the compiled C PCFG will be able to make use of it.