More descriptive and scalable help message
Implement "link mode"

In this mode, when --report-uri1 and --report-uri2 are provided on the
command line, salmonella-diff won't write install and test HTML
outputs for the differences -- it'll link to the salmonella reports
instead.
Don't write nor link test pages when test don't exist

Just show "No test" instead.
Show "No test" instead of "-1" for test status when egg has no test