Syntax guide: Fix typo
Update definition of bottom-packed list-item-line-sequences

Earlier, when a list ended without a blank line (either because the next
list started immediately, or because the parent line sequence ended),
the last list-item-line-sequence was always considered as bottom-packed,
and its last paragraph was not wrapped in `<p>` tags.

This commit fixes this problem.
Automatic links: Fix regexp for detecting email addresses as per RFC 5322

According to RFC 5322, an email address is of the form `local-part@domain`,
where both local-part and domain should not contain: `()<>[]:;@\,."` and
ASCII whitespace / control characters.

We add `` ` `` to the list so that code-spans get recognized with a
higher priority.

We also don't worry about the obscure cases (quoted-strings and obsolete
addressing).
Emphasis: Treat symbols the same way as punctuation

So that `<u>*blah*</u>` is correctly interpreted as emphasis.
A more detailed suggestion for implementers regarding HTML-aware end of paragraph
Identifying reference resolution blocks should handle images within the id

For example, if I want an image inside a link, I would write:

    Here's a [![picture]] you can click to go to my site.

    [picture]: path/to/picture.png
    [![picture]]: http://mysite.com

Previously, unescaped `[` and `]` inside a ref meant that the second
ref-resolution block in the above example was not even identified. This
commit changes that to allow for images to be present in the ref-id of
a ref-resolution block.
Simplify handling of reference resolution blocks

By reusing info we inferred when identifying the block-element
start line of the reference resolution block.
Move null-block section up
Move reference-resolution-block section up
Remove HTML comment that doesn't play nice with redcarpet

Looks like HTML comments within lists are not handled correctly
in Redcarpet now.
Cross link "identifying blocks" rules from respective "interpreting blocks" sections
Fix HTML span element by adding a close tag
Syntax guide: Image: Be explicit on requirements regarding escaping
Syntax guide: Be more explicit on including HTML blocks within list items
Syntax guide: Comments should stay contained within a block