Config per instance

Ref #8

`csswring.wring(css[, opts, conf])`:

    var csswring = require('csswring');
    var processed = csswring.wring(css, {
      map: true
    }, {
      preserveHacks: true,
      removeAllcomments: true
    });

`csswring.processor([conf])`:

    var postcss = require('postcss');
    var autoprefixer = require('autoprefixer');
    var csswring = require('csswring');
    postcss()
      .use(autoprefixer.postcss)
      .use(csswring.processor({
        preserveHacks: true,
        removeAllComments: true
      }))
      .process(css, {map: true});

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