Add 'configtest' option to agent to verify config and exit

Add a '-t' / '--configtest' option which verifies syntax for configs and exits.  This uses stdin input and stdout output after loading input and output (for syntax verification) to proceed with loading and verifying filter syntax. 

This has caught the syntax mistakes that I've made so far, and can happily be used on configs that do not have input or output to greatly speed up the syntax check.

Thanks,
-darren