Support markdown documents

[jlewi]

• Use VSCode's markdown notebook extension as the basis for markdown support
• To support markdown we define a new new notebook type foyle-notebook-md and then register another copy of the controller for it
• The reason we need to register a new notebook-type is because serializers appear to be linked to the notebook-type. Within the serializer it didn't seem like we could easily get the original filename extension to determine what the encoding was.

Fix #19

[netlify]

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