Order of links in extra_nav_links

[rvernica]

I provided a few links for the extra_nav_links parameter. It looks like the links are in random order (probably key-hash order). It would be nice to be able to control the order. Alphabetical by key name might be a good start. A better option might by to allow list (instead of a dictionary) of (key, value) pairs and preserve the order in the list.

[bitprophet]

Yea optionally allowing a list (or if across a backwards compat boundary, simply changing it to be so) seems like the way to go. I've no time to do this myself but might be able to merge a PR.

[joealcorn]

@rvernica I've had success using an OrderedDict to maintain the order of my links. I've opened a PR to document this (#127)