An option to use opts.file rather than opts.data

[danreeves]

It appears custom imports don't currently play nicely with opts.data, and it doesn't pass the correct values to the importer. It works fine with opts.file however.

It didn't like this option was already there. Any way to do this or any reason it shouldn't be there?

e.g.
With the code:

return gulp.src(sassGlob)
.pipe(sass({
    importer: function (url, prev, done) {
        console.log(url, prev, done)
    }
}))

It should output

// url: modules/* 
// prev: /path/to/your/assets/css/main.scss 
// done: function done(data) {
//        bridge.success(data);
//       }

but it outputs:

// url: modules/* 
// prev: stdin 
// done: function done(data) {
//        bridge.success(data);
//       }

[danreeves]

danreeves@4233840 is a working fix with all tests passing. Let me know what you think.

[xzyfer]

Addressed by #353