Support multi-line tips and warns

[linkeo]

Add support for multi-line tips and warns. Implement for #689 .

I did not find a way to add custom block elements to marked, so I just modified helper function to reach the aim.

Changes:

1. Because multiple lines may contain multiple paragraphs or other block elements, I changed container to div, and modified styles about it (and fix margin of the first or last child in the container).
2. Use RegExp to remove !> or ?> at the beginning of every lines, then use a default marked to parse into html content.
3. Add package "he" to decode html to markdown (inline elements will not be reverted to markdown, but it's equivalent to write inline elements in html form)

[QingWei-Li]

Thanks Keo!

BUT I will refactor this feature in v5.0

ref #413