Correct processing of backtick code block on blockquote (fix Issue#2969)

[seaoak]

What does it do?

Fix #2969

This patch makes the filter "backtick_code_block" to remove '>' characters at the head of each line.
How many '>' characters should be removed?
It is decided by the number of '>' characters at the first line.
This patch works well even if there are too few '>' characters at the head of a line.
(Simply removes all '>' characters less than or equal to the number of '>' characters at the first lines.)

Note that this patch works well for a "lang=dos" or "lang=cmd" code block.
The body of these code blocks is usually starting with '>' (DOS prompt).
This patch keeps it if it follows '>' characters which is the same number of the first line.

For example,

> Hello
> 
> ```dos
> > dir
> ```

is rendered to:

<p>Hello</p>
<blockquote>
<pre><code>&gt; dir</code></pre>
</blockquote>

This PR is an alternative of the PR hexojs/hexo-util#110

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage increased (+0.005%) to 97.283% when pulling deaad6b on seaoak:feature/correct_filter_backtick_code_block_on_blockquote into 79bdc95 on hexojs:master.

[curbengh]

I can confirm it also works with

> > ```html
> <b>test</b>
> > ```

[curbengh]

also confirmed it works with,

``` cmd
> ipconfig
```