Templating/layout ignore lists

[alterae]

It should be possible to tell mksite to not do templating or layouts for certain files. Something like the exclude field in Cargo.toml, with globs. Useful for #1.

[ignore]
template = [ "**/*.{png, pdf}" ]
layout = [ "blog/my-special-page.html" ]

[alterae]

Partly implemented, but only with literal paths and not globs