Set postcss to use autoprefixer

[nsrosenqvist]

I'm not sure what version of autoprefixer this filter was created for but according to the autoprefixer documentation it should run as a postcss plugin. The argument passed to the construct should be the path to the postcss executable and an argument needs to be set for it run with the autoprefixer plugin. I didn't update variable names to reflect the executable name change but I did add the argument needed for the command so that it works properly.