Added mod files.
Implemented and tested some basic line parsers.

Currently only able to tell the difference between text lines and empty
lines.
Added and tested code to combine lines together into blocks.
Merged the two element files into one.

Refactored BlockCombiner slightly.
Removed unnecessary EndOfInput option in BlockCombiner.
Fixed pattern match in LineParser to be exhaustive.
Trim whitespace off of text lines.
Added line parsing for both varieties of headers.
Added support for headers to the block combiner.
Moved MarkdownElements into parser, as that's the only place it is used.
Implemented & tested the span parser to parse out links.
Minor refactoring to make EmptyLine not a MarkdownBlock.
Removed now-unneeded test code.
Finish parser
Added the Guide item. Does nothing at the moment.
Implemented the basic structure for displaying a GUI when the guide is
used.
Finish basic-gui
Added images and code to render the basic GUI for the guide.
Finish real-gui
Added basic skeleton of the render tree.