allow the `el` option to accept an `HTMLElement` instance

[trusktr]

Summary

The el docs say

It can be a CSS selector string or an actual HTMLElement.

which wasn't true. This change updates the code to match the docs.

Additionally this allows Docsify to be embedded in custom elements or components systems like React/Vue/Solid/etc where normally people have a reference to an element.

What kind of change does this PR introduce?

Bugfix? Feature?

For any code change,

• [x] Related documentation has been updated if needed
• [x] Related tests have been updated or tests have been added

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

Related issue, if any:

Tested in the following browsers:

• [x] Chrome
• [ ] Firefox
• [ ] Safari
• [ ] Edge
• [ ] IE
• [x] jsdom

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[trusktr]

I realized that dom.getNode() has similar logic as what I implemented here. Using that would make this PR shorter.

• [ ] use dom.getNode instead