(MODULES-1582) Initial spike for % placeholder

This simply `gsub`'s the file path into where the % placeholder is.
(MODULES-1582) Specs for the new % placeholder

These specs are pretty much the same as the originals, but now check that the output has the correct replacement for file location
(MODULES-1582) Improve % detection

Avoids any validate commands that have %'s in them other than "... % ..."
Merge pull request #377 from petems/MODULES-1582-improve_validate_cmd

(MODULES-1582) File location placeholder