ability to specify the `nav` element

I'm trying to embed docsify within an existing template, but I'm having a problem where docsify is hijacking my template's `` tag.

Looking through the source, it seems like docsify [chooses the first ``](https://github.com/docsifyjs/docsify/blob/master/src/core/render/index.js#L404) on the page.

Is there any chance we could get a `nav_el` config option that lets us specify the id of the `` element we want to use?