Output to either a single file or one file per-module

[criccomini]

This PR adds the ability to output to multiple files.

If output_path points to a directory instead of a file, the renderer will now output one file per-module in a flat structure like so:

index.md
package_name.md
package_name.foo.md
package_name.foo.bar.md

The index.md file contains a table of contents with links to each package .md file.

I also updated the Markdown styling quite a bit to make it easier to read.

Fixes #2