Support for multiple .literate document projects

[jesterKing]

Design and implement a good mechanism to support multiple .literate document projects.

Whilst this is with multiple .literate document projects in mind the feature should work equally well for single .literate document projects.

• TOC
  • TOC per book
  • TOC per chapter
• Cross-referencing
• Inlining of code for fragments in generated .html

[jesterKing]

Since v0.2.1 the extension manages multi-root workspaces better. This doesn't mean multiple literate projects with cross-referencing, but rather that projects in different folders can be added to the same workspace without these interfering with each other.

Still, good cross-referencing between projects would be an interesting excercise.