Add option to retain testing URL's directory structure

[kftsehk]

Some may prefer to put reports into URL's original directory structure

New behavior

http://example.com => $OUT/example_com.report.json
http://example.com/ => $OUT/example_com.report.json
http://example.com/?a=b => $OUT/example_com/_a=b.report.json
http://example.com/a/b/c => $OUT/example_com/a/b/c.report.json

This can be enabled with --retain-url-directory