Namespace EP content zones so we can apply styles to links, etc.

One of the big challenges of this boilerplate is any styling we apply has to be namespaced so it doesn't leak into an end user's patterns and styles.

For the most part we do this with lots of classes starting with `EP_`. For example, we have `EP_nav` for our nav styles. This works well for _HTML_ we're writing, but not so well for markdown.

Markdown is processed and output without any special classes so it's difficult for us to apply styles to these content sections. We should wrap content sections in an EP class so we can style elements within them. For example we could have an `EP_prose` class and then write CSS like this:

```
.EP_prose a {
/* link styles */
}

.EP_prose ul,
.EP_prose ol {
/* list styles */
}
```