[iOS] Fire event when span within an a-label is clicked.
[iOS] Fire event when span within an a-label is clicked.

* feature: If a span is clicked which is in an a-label of a richtext component, an itemclick event will be fired.
* demo: http://editor.weex.io/p/sunshl/Contribute/commit/b21e1133830b48767c6d00d712e415b2
* doc pr: apache/incubator-weex-site#362

alibaba/weex,alibaba/weex,alibaba/weex,alibaba/weex,alibaba/weex,alibaba/weex,alibaba/weex