Lazy yaml support

[yhatt]

This PR will add lazyYAML constructor option to support lazy YAML format.

Motivation

Directives like to specify color code are sometimes looking like not working. A major cause is that YAML parser recognizes # as a comment by following YAML 1.2 spec.

backgroundColor: linear-gradient(to bottom, #fff, #000)

As you can see from this highlight, YAML parser will recognize a string until reached #, and it treats fff, #000) as comment.

Wrapping by quotes is a way of around this mis-recognize, but we recieved some negative feedbacks from user of private alpha test.

Logic

We use a heuristic logic to parse YAML when Marpit class is initialized by lazyYAML: true.

When you are using only supported directives by a plain format like a pair of {key}:{value}, we treat a written text after semicolon as value directly.

If we detect unknown key or allowed YAML special char, we will fallback a parser to strict YAML.