instead of returning false for options[:exit_on_warn]

instead of when reporting, which lead to confusion
and filtering all over the place instead of in one place.

although we're still depending on "globals",
just not ones shared with every Ruby app

for modularity and easier reuse by external libs

--no-progress turns it off

and Brakeman.run should return the tracker

Put exit code constant in Brakeman namespace; use Checks#all_warnings, not #warnings ; don't steal the -s option

allowing Brakeman to easily be used as a library now

-n option will run the checks sequentially.
Closes #10