A progressive Markdown rendering module.

[quank123wip]

Idea: make 2 rendering process
First, render markdown text as plain html using markdown-it.
Second, add a customized rendering process convert the plain html to a themed, more rendered(features like inserting videos will be achieved at this process), and prettier html using a module need to be implemented here.
To-do List

• markdown-it with extra modules
• customizable html renderer
• theme on final html which can be switched by user

@undefined-moe for reviewing this idea.