Doesn't do the same thing as `mapFile` option of gulp-sourcemaps

[phated]

I was trying to integrate this into gulp-sourcemaps and realized that this module isn't do the same thing as the mapFile option. The option actually changes the sourcemap's filename which affects the sourceMappingUrl comment and the path of the sourcemap vinyl object.

I'm not sure how to make a module that does those things as a separate stream while also keeping backwards compatibility.

@nmccready @floridoo any ideas?