Updated imacss version to 0.3.0

Just did a test on generating custom css selector with gulp, it worked as expected.
```
gulp.task('mytest', function () {
    gulp.src('./files/images/new/**/*.svg')
        .pipe($.imagemin({
            svgoPlugins: [{removeViewBox: false}],
        }))
        .pipe($.imacss('_test.scss', function(image){
            return '.svg-' + image.slug + ' { ' + 'background-image:' + 'url(\'' + image.datauri + '\'); } ';
        }))
        .pipe(gulp.dest('./files/sass/'));
});
```