Add support for include/exclude options

[mdouglass]

I would like to add options to the configuration file include/exclude that allowed you to specify a set of globs to scan for markdown files (similar to the way include/exclude work in TypeScript's configuration files). If a file is found, we would read the YAML frontmatter at the start of the document and used that to obtain the pageId/title values.

We have a large monorepo and avoiding having a central list of documents would be nice to have. Admittedly, we could generate the pages list on the fly during the build if we had to, so I understand if you aren't interested in this feature.

That being said, I'm happy to do the work for it -- I just want to make sure you're open to the PR before I start on it.