Allow user to disable CSS output source map

[hochhaus]

While updating some of our code to use rules_closure 0.3.0 I ran into google/closure-stylesheets#11. Basically, the CSS compiler crashes with a NullPointerException when source maps are enabled if a property value contains a comma. For example:

body {
  font-family: 'Roboto', sans-serif;
}

Unfortunately, for me, this is a show stopper for the upgrade. I don't see an easy way for me to disable the source map in the current API. Can we provide an override until this is fixed in the CSS compiler?

[hochhaus]

@jart Any time to offer feedback on this?

[jart]

I'll try to get a release out today fixing this.