Support setting background color by markdown image syntax

[yhatt]

Support setting background color by markdown image syntax ![bg](#abc123).

If an image URL is parsable as CSS color, Marpit would assign it as backgroundColor directive, not backgroundImage. This behavior has no difference between in regular mode and inline SVG mode. We're using color-string package to resolve CSS color (and Marpit can also parse currentColor additionally).

For example, below 2 markdown files have the same output.

 # Hello

<!-- _backgroundColor: red --> # Hello

This PR includes the refactor of a bit bloated Marpit background image plugin to be modularize into multiple plugins. It resolves #92 and yhatt/marp#252.

ToDo

• Add docs
• Add more test cases
  • Regular mode
  • Inline SVG mode