Incorrect anchor link generated when there is a url link in the header

[fresheneesz]

For the header:

#### Instance properties inherited from [`EventEmitter`](http://nodejs.org/api/events.html)

doctoc is generating:

[Instance properties inherited from [`EventEmitter`](http://nodejs.org/api/events.html)](#instance-properties-inherited-from-eventemitterhttpnodejsorgapieventshtml)

Which doesn't link to the header (tho it does link to EventEmitter)

Related issue: thlorenz/doctoc#57

[thlorenz]

We need to ignore [ and ] from links it seems.
Could you please confirm and provide a PR with a fix?

Thanks.