Add a link to a check

Hey Mark. It would be useful if one could attach a URL to a check.

I usually find that when an alert goes off, after seeing a worrying graph in Seyren the next step is to jump to some other URL to understand the problem better (e.g. a logging app to see what what's being logged, a detailed graphite graph with multiple targets to see other things that are going on, etc). It would be great if this link was attached to the check, since it would be convenient for me but also super useful for someone else who needs to understand the check better (and understand what to look at when it's alerting).

Happy to submit a PR for this but I thought I'd see what you think about the following options:

1. Add a **Link** field to the check details, the value is assumed to be a URL

2. Linkify the existing **Description** field with some js, so that links can be added there and they will become anchors

3. Support markdown in the description field with some js, so that links can be added there like `[logs](http://myloggingapp/...)` and they will become anchors

Obviously option 1 is most limited, but it would allow the **Link** to be included in alert emails. Option 3 might be a bit excessive... but markdown.