Added a --crash flag to the agent.

The original goal was to do this on sigusr2, but installing that handler caused problems with crash dumps.  I'm not sure where the real problem is (luvit/breakpad).  

This will allow us to test crashes in official builds and the whole crash dump pipeline.  I'm not really happy with were this code lives, but it has to run after we have fully initialized our lua/luvit stack.