Support multi-line quote block ending with a blank line (GFM)

According to [GitHub Flavored Markdown Spec 5.1](https://github.github.com/gfm/#block-quotes), a quote block can end in a blank line, as illustrated below:

```
> # Foo
> bar
baz

```

this is [Example 210](https://github.github.com/gfm/#example-210) of the official document.

However, if the regex pattern of vim-markdown-toc does not accept this.