Indicate source file of errors and warnings when generating documentation

The warnings are great but it's hard to tell where they might be coming from. Patch attached to emit source file names for errors

• creates an 'InitialRenderContext instead of just a RenderContext, and uses that to initialize the DocEngine so properties are always available.
• adds "sourceFile" property to context and updates for each file being rendered
• adds 'emitWarning' for consistent messages
• uses constants for context property keys
• removes custom configureContext field from DocEngine