Markdown page app

[preaction]

We need an app that does simply Markdown pages.

It should probably handle the same YAML format that the Blog does (i.e. the default Statocles::Document).

[preaction]

With this one, I can see there being some effort to make sure that other directories are not built, as it looks for the same documents as other apps (the blog) do.

I don't know if I want to explicit-include or explicit-exclude. Probably the former, since it introduces the problem right away and leads to fewer wonders.

Then, the app could update its own configuration...? Does Beam::Wire know how to save itself as a file? No. That might be an interesting feature...

[preaction]

It'd be nice if we could, in the future, use this as the "default" thing if no other app has claimed a document, but that sounds like a bigger undertaking.

[preaction]

With preaction/Beam-Wire#37 we could use the app to update its own configuration.