Set background color by markdown image syntax

[yhatt]

Related to yhatt/marp#252.

Marpit is already supported backgroundColor directive. But when you want to change color in a single page, it requires:

• A knowledge of difference between local and spot directives
• A lot of typings
• Strict YAML formatting

<!-- _backgroundColor: "#def" -->

A suggestion in yhatt/marp#252 is very useful in this case. You may write the color code as like as the background image syntax.

<!-- Color keywords -->
![bg](red)

---

<!-- Hex color -->
![bg](#39ffaf)

It would allow color keywords, transparent, currentColor, and RGB colors with hexadecimal notation #123abc. It has the same behavior to backgroundColor spot directive.

The functional notation like rgb(0, 128, 255) might break Markdown (CommonMark) image syntax by whitespace. To support functionals is no problem, but we should tell this problem to user.