Autoprefixer option

[Andrew Abramov]

Add autoprefixer option to css-stylus tech. Need support browsers config.

Boolean | {{ browsers: String[] }}. The default value is false.

• If false — do nothing.
• If true — autoprefix with default browsers or config.
• If autoprefixer.browsers — autoprefix with specified browsers.

[Vladimir Alaev]

Closed by #77