Added preprocess step for response files.

$set Name=Value
$if Name=Value
$endif
Update mgcb.md

Documented /set, $if, and $endif
Merge branch 'develop' into PipelinePreprocess

Conflicts:
	Documentation/mgcb.md
Merge pull request #2669 from SickheadGames/PipelinePreprocess

Added preprocess step for response files.