Clone slack's code snippet feature.

I think this is different from
https://mattermost.uservoice.com/forums/306457-general/suggestions/12927891-code-snippet-with-preview
Which gives more detailed explanation at the ends.

It is also a little different from
https://mattermost.uservoice.com/forums/306457-general/suggestions/15495801-add-a-one-click-action-to-create-a-new-snippet
In that not requesting to do away with fences, just want the slack snippet feature.

Thanks!