Reinstate support for looking out for specific signatures in stacks

Rev 44385808826ed7e7f7b16c7d4b57c46a43c5e0d0 removed support for looking out for specific signatures in stacks, we should look into reinstating it, especially when moving to Python 3 in the future.

i.e. we used to be able to support --sig=<some signature> via crashes.py, and we should support this again, going forward.