Reduce inconsistency about html option between Marp and Marpit

[exKAZUu]

Marp constructor ignores markdown.html option (c.f. https://github.com/marp-team/marp-core#html-boolean--object), but, Marpit constrcutor accepts markdown.html option (c.f. https://github.com/marp-team/marpit/blob/master/docs/usage.md#-markdown-it-parser-setting)

I feel this structure is a bit confusing. Could we unify markdown.html v.s. html ?

[yhatt]

Transferred issue from Marpit ▶️ Marp Core 🚚

Surely it's confusable. Internally Marp Core's html option has already become just an alias to markdown.html by working at #74. It may be better to unify interface into markdown.html and match to Marpit.

Background

Marp Core has an extra feature that there is not in Marpit, to sanitize HTML by whitelist if JSON was passed to html.

The reason why splitted options into html and markdown.html in Marp Core was afraid breaking compatibility with the other markdown-it plugins, because typically markdown-it's html option does allow boolean value only.