Deprecate old paths.markdown config option

The old configuration option `paths.markdown` has been avoided for a
while (ever since a9220de83ed1fc3b75012e04ce0e787c5c107f3a)

Deprecation warning emitted as a grunt error, but does not stop the
build (or require --force)

Next step is to promote to grunt.fail.warn and finally remove it
altogether.