Allows inline & other supported map options.

There was a bug where gulp-autoprefixer would remove inline source maps from my gulp-sass plugin since it doesn't produce external sourcemaps. So now it can check for the option and use that value otherwise do what it did before.

So you can send:

```
autoprefix({
     browsers: ['last 2 versions'],
     map: 'inline' // Or any support autoprefixer-core processor -> map variables
})
```