Add <progress> support

[Mittineague]

Exactly. I see no reason for this to be in Core when it is probably one of the easiest Plugins there could be. eg,

plugin.rb

# name: progress
# version: 0.1 

register_asset "javascripts/progress.js.es6", :server_side

progress.js.es6

Discourse.Markdown.whiteListTag("progress", "class", /d-prog/);
Discourse.Markdown.whiteListTag("progress", "max", /[0-9\.]+/);
Discourse.Markdown.whiteListTag("progress", "value", /[0-9\.]+/);