Remove need for node when generating source maps about css

[conradz]

Yeah, I think we should have an option to disable applying original source maps (this is to convert source maps of generated input files, such as Sass, to the output source map). Will work on this.

[ai]

Best settings is no settings :). Maybe if ( fs.readFileSync ) { … } check will be better? Like in PostCSS

[conradz]

The reason I added an option for this is so that (a) behavior does not unexpectedly change when running in browser and (b) this option may be useful for people who do not want this but are running it on Node.

[harthur]

As someone using it, I'm fine with specifying a setting, and to me that seems like it would be better, fwiw.