Add styling options for markdown

[DemonZito]

• A markdown seed can be added with markdown.New but has no styling.
• Allow developers to add their own css styling.

Example, H1 and H2 are currently rendered as this:

[Splizard]

This is caused because the Qlovaseed app package introduces a CSS reset that sets many styles to a default property. I did this to help ensure cross-browser consistency. However, I believe that there should be either a default set of reasonable styles for Qlovaseed apps and/or a way to recursively style seeds and their children with set.Style options.