ENH: Add ability to specify custom icon images

[choldgraf]

Description

This adds the ability for people to specify local images for their icon links, instead of only allowing fontawesome icons. Here are the major changes:

• Adds a type field for icon link config. The allowed values are type: "fontawesome" and type: "local". fontawesome is the default, so if people don't specify type, it'll default to the current behavior.
• Adds type: "local" and then supports icon: path/to/img.png, which will resolve to an img element instead of a span.
• Adds a helper if an incorrect type is added to tell people it's configured wrong.
• Adds some testing for icon links because we weren't testing it before.
• Adds a demo of this to our documentation with the pandas logo

Here's how it looks in our demo docs:

Rationale

My rationale for adding this is that projects may want to link out to a "parent site" with this theme. For example, the JupyterHub docs might want to link up to jupyter.org. In that case, we could use an icon link with the Jupyter logo, rather than using extra screen real estate by putting jupyter.org in the navbar links.

[damianavila]

Left some comment/suggestions that you already incorporated, so LGTM!

[choldgraf]

great! thanks :-)

[jorisvandenbossche]

Nice feature, thanks!