Initialize wx style processor

@inner
@param {Object=} opts the processor init options
@param {Array=} opts.plugins the postcss plugins
function initStyleProcessor(opts) {
    registerProcessor({
        name: 'wxss2css',
        // using the existed postcss processor
        processor: 'postcss',
        extnames: ['wxss'],
        rext: 'css',
        options: opts || {
            plugins: [
                wxssPlugin
            ]
        }
    });
}