Allow event_link to also pull event data

[terraboops]

I'd like to extend the event_link shortcode to optionally pull from the event data - so that I can pull cfp_link and registration_link from my data file and link to them.

This PR adds two new properties to the shortcode, while preserving existing behaviour:
url-key - specifies a key whose value will be bond to the href of the link
text-key - specifies a key whose value will be bound to the text node of the link

If both url-key and page are specified, url-key will be preferred. Similarly, text-key will be preferred over text.

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This change is 

[mattstratton]

Interesting! Can you do me a favor and open an issue that will connect to this PR, explaining it? The reason is that our changelog generator is driven by closed issues, and I want to make sure that this gets credit if we merge it!

[mattstratton]

@tylermauthe - two things:

1. can you open an issue in this repo as I mentioned above so we can link this as a new feature?
2. Can you put an example into 2017-ponyville so we have a test?

[terraboops]

Sorry for the delay - I've created #623 and added the example as requested 🙌