Proposition for improvements to note plugin

There are several propositions:
1. Add more note types with more color options (e.g. grey)
2. What about removal on title attribute for note. It is optional, but when it is absent it still renders as empty block.
What about going from this syntax:
```
{% note title="title" %}

content

{% endnote%}
```
To this:
```
{% note %}

*title*

content

{% endnote%}
```