allow pages to be called by filename

Now you can call a page by it's filename, like this: `{{ pages['page'].title }}` instead of doing a `foreach` with a `if` statement inside it.

I find that breaking out the content into smaller sections is way more flexible than having to render out an entire pages content.

This way I can have many small files that fill out different sections of my theme/site.

I can have a footer.md or a header.md. It's almost like having "widgets" in Pico.

*Side Note:* this doesn't let you call the homepage (content/index.md) because it needs to be the `$pages[0]` page.