[feature request] support page layout

[ulivz]

Motivation

Currently if we want to custom the page's layout (e.g. hidden sidebar, toc and pageEdit), we need hidden it via frontmatter one by one:

---
toc: false
pageEdit: false
sidebar: false
---

And after #42 is merged, the page' container style is not aligned and is hard to control, it's not friendly for users who only wants to use a fully different page layout and only remain Header.

API Design

Introduce a new frontmatter option BlogPage:

---
pageLayout: BlogPage
---

The the page will be rendered with the component, and we don't need to hack any internal styles.