Allow users to disable quoting completely or use a different quote string

Allow users to disable quoting completely or use a different quote string.

This is needed to fix `grunt-contrib-sass` as the current issue there is only solvable by removing quoting completely. Otherwise it will fail under Linux and OSX.
I know. But strangely enough it works in some cases. Such as in `grunt-contrib-sass`. It might be handled in `spawn`.