Implement background image sizing with keyword and scale

[yhatt]

Support assigning backgroundSize spot directive in Marpit background syntax. It can use the keyword value of background-size style, and fit keyword that is a compatible syntax with Deckset. Of course it can specify a percentage as scale.

![bg contain](https://example.com/bg.jpg) <!-- Fit to slide size -->
![bg fit](https://example.com/bg.jpg)     <!-- Compatible syntax with Deckset -->
![bg auto](https://example.com/bg.jpg)    <!-- Keep original image size -->
![bg 125%](https://example.com/bg.jpg)    <!-- Specified scale -->

NOTE: The pre-release version of Marp is supported inline image scaling. It uses zoom style but it has not supported in Firefox. Marpit's static HTML output requires cross-browser support.

In addition we do not want to change DOM tree for this. Marpit's theme CSS would become to not clear markup. (PostCSS plugin might resolve it)

ToDo

• Add tests