Operators for class directive

[yhatt]

For now, class directive is just meaning setAttribute('class', 'xxxxxx') so assign the specified value into the slide as is. The problem is removing inherited class unless re-assign the class manually.

When setting the whole of color scheme by class, it may lose the color if changed the layout in a part of slide via _class spot directive.

<!-- The example based on Marp Core -->

<!--
theme: gaia
class: invert
-->

# Invert color

---
<!-- _class: lead -->

# Invert color?

---

# Invert color

The second page has lost invert color.

Proposal

Marpit recognizes + and - prefix in each of class names, and manipulate inherited class based on the operator. Don't require to know the inherited class.

• <!-- _class: +lead --> adds lead class into the inherited class.
• <!-- _class: -invert --> removes invert class from the inherited class.

It works on local directives and spot directives.

By this change, the class name beginning from specific operators cannot use in Marpit. Beginning + and - are valid as HTML class but not common because + requires to escape character in CSS definition.