Add macro which uses pre-existing buffer for rendering

[StratusFearMe21]

This PR adds the html_render macro to this crate which produces a struct that is able to both render to a pre-existing buffer, and also be passed into another invocation of html and render inside of that without allocating a new buffer.

Here's the documentation I created for the new html_render macro:

Functionally the same as [html] but produces a struct that implements [Render].

You can

• Call render() to render the HTML as [Markup]
• Call render_to() to render the HTML to a pre-existing &mut String
• Pass this directly into an [html] invocation to add it as a component without allocating a new buffer.

use maud::{html, html_render, DOCTYPE};

let component = html_render! {
   p { "Wow, such component!" }
};

let full = html! {
   (DOCTYPE)
   head {}
   html {
       (component)
   }
};

This PR resolves #381 and #90

[StratusFearMe21]

The performance wins are indeed significant