when onlyCover is true, also provide an option to show the menu.

[trusktr]

Continuing from #1173 (comment)

Feature request

When only onlyCover is true, it would be great to have an option to show the menu (and of course the menu should not be empty).

What problem does this feature solve?

Allows people to skip to content faster.

What does the proposed API look like?

window.$docsify = {
  onlyCover: true,
  onlyCoverShowMenu: true, // false by default
}