Dark mode support

[yhatt]

This issue came from the result of sorting out #138.

Windows 10 and macOS Mojave are supported the dark color scheme. We are planning support dark mode at Marp Next.

An issue is how to treat color scheme of slide. Marp (Marpit) markdown returns immutable result from same Markdown. If the default color scheme is changed by dark mode, the output become to change by user environment.

I thought that is not good behavior, but user may expect to keep mode's color scheme by default.