Add support for most common nested elements in block quotes

[rmecham]

I've had a go at updating Ink to support what I think are the HTML elements most commonly nested inside block quotes. I'm relatively new to Swift, so please let me know if you have any comments or suggestions.

[mflint]

I was excited to see your contribution @rmecham. I've tried this with an example from Gruber's markdown syntax page:

> ## This is a header.
> 
> 1.   This is the first list item.
> 2.   This is the second list item.
>
> Here's some example code:
>
>    return shell_exec("echo $input | $markdown_script");

This renders as two separate blockquote elements, but I'd expect it to be just one.

[mflint]

Thinking aloud: The PR also adds explicit code for other fragments inside the blockquote, checking for #, -*+ and > - but there's already support for that when parsing the document in MarkdownParser.swft:67.

Ideally that code would be reused - but it might need part of MarkdownParser to be extracted into a Document fragment, so BlockQuote can instead do:

let nestedDocument = try Document.read(using: &reader, ignorePrefix: ">")
blockquote.items.append(nestedDocument)

The ignorePrefix thing is smart. ❤️

[russellgordon]

I'll chime in and express my enthusiasm for this pull request. In particular, the idea of multiple > lines being rendered as a single block quote, such as:

> 1.   This is the first list item.
> 2.   This is the second list item.
>
> This renders as two separate blockquote elements, but I'd expect it to be just one.

In fact we can see that's just what the GitHub markdown parser does, if you inspect the following element, you'll see it is transformed to a single blockquote:

1. This is the first list item.
2. This is the second list item.

This renders as two separate blockquote elements, but I'd expect it to be just one.

I had to do some gymnastics with inserting <hr/> tags and CSS to get a block quote to render visually across multiple lines. That's not ideal and it'd be nice to see Ink more fully support block quotes in this manner.

[kevinrenskers]

This is definitely something that is needed in Ink.