Feature grade

Added -grade which automatically uses -json-flat and -rawoutput to show only the grades, e.g.:
> ./ssllabs-scan -quiet  -grade -hostfile=./tf 
"testing1.yoursite.com": "A"
"testing4.yoursite.com": "F"

NOTE: I've been maintaining this separately, but more than a few people sent me nastygrams after your latest flurry of fixes because they can't use the new stuff with -grade, so I figured a merge could stifle the noise.