Support resuming failed runs

[jvilk]

While BLeak works well on most applications / websites that I throw at it, others throw curveballs that often turn up during the diagnose step.

BLeak should serialize an early version of bleak_results.json to disk after the leak locating / ranking stage. If it sees an incomplete bleak_results.json in the specified output folder, it should use it to resume at the diagnose phase.

This will also greatly reduce user frustration if BLeak bugs out on their website after a long run.

[jvilk]

BLeak should properly do this now, but I haven't tested the functionality yet. :-)