Add option for appending sourceMappingURL comment

[jessepollak]

Right not, if libsass is used to generate source maps, it automatically appends the sourceMappingURL to the CSS file. This causes issues where additional processing and/or relocation of the source map file is done upstream in the build process.

An example of where this causes problems can be found in dlmanning/gulp-sass#51. There, we need to strip the comment so the upstream processes can add it back in correctly.

It would be great to have an option to decide whether this comment is appended automatically or not.

[am11]

+1, exactly what we are looking for in node-sass.
sourceMapURL: false option should skip appending URL in CSS even if the source-maps are generated separately.