Allow diagnostics messages to have markdown (or formatted text) content

[mjbvz]

Feature request
Allow diagnostics to display formatted content. The specific request was to show part of a diagnostic message in bold.

A few potential options:

• Allow diagnostics to use markdown content
• Allow diagnostics to use formatted text
• Add an "important span" to diagnostic messages that lets us control the styling of the diagnostic.

(This request came out of the FB meeting)

[octref]

Also related: vuejs/vetur#849.

[ChuckJonas]

At the very least it would be helpful to have to ability to link them to an external url (EG: web resource explaining the cause of the problem)