Allow previewing message draft

[nesium]

When composing larger messages it would be handy to see a preview of the rendered version of the message. I'd suggest that we implement this like here in Github issues instead of having a WYSIWYG editor like Slack.

Let me know if you agree @valeriansaliou, so I can add a method to transform a Markdown string to HTML.

[valeriansaliou]

This sounds good to me, definitely not liking Slack's WYSIWYG editor so I'm open to a GH-like editor.

[nesium]

ProseClient now contains

previewMarkdown(markdown: string): string;