fix(highlight): code in blockquote

[curbengh]

Currently, when inserting a codeblock inside a blockquote, e.g.

> ```json
> {
>   "test": 123
> }
> ```

results in,

> {
>  "test": 123
> }

---

Expected behavior is

{
 "test": 123
}

This fixes hexojs/hexo#2969 & hexojs/hexo#3318.

However, this fix doesn't apply to cmd or dos codeblock because the line may start with >, e.g.

> ipconfig

cmd codeblock in blockquote,

> ```cmd
>> ipconfig
> ```

would result in,

> ipconfig
>

A workaround is to remove > from the closing backtick,

https://pastebin.com/s94kaD4j

which should render as,

> ipconfig

or use {% blockquote %} tag plugin.

[coveralls]

Coverage increased (+0.03%) to 95.23% when pulling f09d4fd on curbengh:blockquote into b292332 on hexojs:master.

[curbengh]

this is a workaround to remove unnecessary new line from

>```

[curbengh]

<span> exclusion is a workaround to be compatible with sublanguage highlight.

[curbengh]

In regards to https://github.com/hexojs/hexo-util/issues/108, it may be more suitable to put this workaround in backtick_code_block.js, but currently I'm not sure how well prism/atom handle blockquote.

If later we find prism/atom cannot handle blockquote, then this workaround shall be moved to hexo.

[seaoak]

I think a workaround in the hexo (backtick_code_block.js) might be better
because the module is responsible to the extraction of code block (and blockquote).

I found examples of markdown notation in the GitHub Help:

• https://github.github.com/gfm/#fenced-code-blocks
• https://github.github.com/gfm/#block-quotes

There are many examples which current backtick_code_block.js can not process correctly:

• Example 98
• Example 101
• Example 102
• Example 103
• Example 215
• Example 228
• Example 229

As shown in Example 229,
the question "how many '>' at the head of line should be removed"
is only answerable by Markdown parser.
A code-highlighting module is not responsible to this question.
(And also it is not affected by "lang" parameter of the code block.)

I'm trying to improve backtick_code_block.js,
but it is almost like developing small set of "marked".
I started from check the behavior of "marked" about the examples....

[curbengh]

Superseded by hexojs/hexo#3765.