Allow markdown panic guides

[lucas42]

Currently, go-fthealth expects the panicGuide field to be a URL, but the spec says markdown is supported. We should handle markdown correctly when displaying it to users, rather than treating it as a URL.

Example use case: https://github.com/Financial-Times/coreos-version-checker/blob/master/version.go#L26

[tosan88]

Is this still valid or is it resolved as per #10?

[lucas42]

No, #10 will show plaintext copy for panic guides, but doesn't handle markdown.