Allow the use of all autoprefixer options

[nikita-graf]

There is no way currently to pass external stats file to autoprefixer, since everything except browsers property is omitted. I wanted to setup it like so:

[require('enb-stylus/techs/stylus'), {
    autoprefixer: {
        browsers: [
            '> 0.5% in my stats'
        ],
        stats: myCustomStat
    }
}]

This PR solves this issue.

[qfox]

👍

[blond]

@nikita-graf, thanks!

[blond]

Fixed in v2.3.2.