Adding vimeo as an option

[bridgetkromhout]

As reported in #542 it appears we didn't have code in the talk page to handle displaying vimeo. I've copied the youtube code and modified it to work with vimeo. In local testing this makes vimeo show up correctly.

This will need to be merged and released before devopsdays/devopsdays-web#2471 works correctly.

Fixes #542

[mattstratton]

Before merging, I need to see if there are any docs that also need updating (reference.md, etc).

I'll review later tonight if possible.

[Roderick-Jonsson]

My heroes :)

[bridgetkromhout]

It looks like this currently is the only item at its indent level:
## Fields in YYYY-CITY.yml

@mattstratton does this mean that you intend the reference.md file to later also talk about frontmatter from other file choices such as a talk file? It's unclear to me where we currently want to explain the talk file frontmatter.

[mattstratton]

Yeah, I guess I never finished that part :)

[mattstratton]

Basically, reference.md should be considered canonical for every field/"api" type thing in the theme. I just haven't gotten to that level yet.

[mattstratton]

I'm good with merging this. I'll add in another PR for this release to include talk page frontmatter in the docs, since we need that anyway.