style(callout): add callout tag

[ghouzenyan]

What kind of change does this PR introduce? (check one with "x")

• Bug fix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

Does this PR introduce a breaking change? (check one with "x")

• Yes
• No

---

Description

When you read the Hexo doc, you would sometimes see a blockquote with a red or yellow border, which is called callout. Here is it.

A custom tag for the Bootstrap callout style is available for material theme.

Usage:

{% note [type:default|primary|success|info|warning|danger] [header] %}
...content...
{% endnote %}

Examples:

{% note warning %}
### H3 in Warning Callout
This is a warning callout.
{% endnote %}

{% note success <h4>H4 in Success Callout</h4> %}
This is a success callout.
{% endnote %}

{% note primary Default Header in Primary Callout %}
{% endnote %}

{% note %}
This is a default callout.
{% endnote %}

BTW, I have modified the original blockquote style to unify theme style.

---

Verification steps

This is the TOC:

[neoFelhz]

You should add a comment just as:

 /****************/
 /* Callout Style */
 /****************/

[iblh]

Good job! but our markdown style is reference Github.
The default blockquote is like this below:

H3 in Primary Callout

This is a primary callout.

[neoFelhz]

@viosey The NexT has this style. And we can add it to the theme docs as well.
Some times we may need it.

[ghouzenyan]

@viosey Ops, I refer to the design of Twitter's. So I've changed out default blockquote style. Maybe I need to learn more about material.

[cubesky]

I think this is Bootstrap style ,not Material style

[iblh]

This's a nice feature, but we need to refactor it's designing to cater to Material.
Take your time.