Parse TOML blocks inside blockquotes

[mgeisler]

If a TOML block was found inside a Markdown blockquote, we would
report a TOML parse error. This is because we extracted the code block
directly from the original text and thus incorrectly included the >
characters.

So parsing

> This is a blockquote
>
> ```toml
> clap = { git = "https://github.com/clap-rs/clap/" }
> ```

would result in an error like this:

README.md (line 22) ... TOML parse error: unexpected character
found: > at line 1 column 1 in

> clap = { git = "https://github.com/clap-rs/clap/" }

To fix this, we now parse the code blocks by listening to the Start
and End events from pulldown-cmark. This should be much more
robust.

Fixes #83.