Merge Markdown and SyntaxHighlighter, and import separately

[alexcjohnson]

Two-part issue:

• @wbrgss has a way to work syntax highlighting directly into Markdown, so we can drop the SyntaxHighlighter component altogether.
• Since the code for these two (or one) components is by far the largest piece of the dcc JS bundle, it would be great to separate that out. My thought is we can require Markdown to be imported separately - ie you can't use it directly as import dash_core_components as dcc -> dcc.Markdown - instead you need to do from dash_core_components.markdown import Markdown, and in the file markdown.py we would modify dcc._js_dist - similar to how dash-cytoscape has load_extra_layouts https://github.com/plotly/dash-cytoscape/blob/541f9a52e25f44f0ad5eb165ba488a9179f847c7/dash_cytoscape/__init__.py#L52-L95

[wbrgss]

Closed in #562