ability to specify the `nav` element

[Evidlo]

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

Looking through the source, it seems like docsify chooses the first <nav> on the page.

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

[Koooooo-7]

not yet, we register selectors just on nav rn. could u give us a simple demo to reproduce it ?
we will check it, thx a lot.

[pauldraper]

related #799

[trusktr]

This will be part of #799. #799 needs a few more things besides the nav fix.