Allow controlled search options

[joeyparis]

Prior to version 3 if nothing (as in null/undefined, not an empty array) was returned for the getOptions prop the component would continue to use the values passed in the options prop. This is helpful when you want the options to be controlled by your own component rather than managed by the SelectSearch component. This update should not affect existing behavior.

Here's a rough example:

const Dropdown = ({ default_options }) => {
	const [options, setOptions] = useState(default_options)

	const advancedOptionFunction = (search) => {
		// ...do advanced search things
		const new_option = () => [] // pretend this gets new options
		setOptions(new_options)
	}

	return (
		<SelectSearch
			search
			options={options}
			getOptions={(q) => {
				advancedOptionFunction(q)
				// Note the lack of return. Returning an array or Promise
				// would still act the same as before this pull request
			}}
		/>
	)
}

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[joeyparis]

I was able to improve test coverage overall, but I took the equal.js code and relevant tests from fast-deep-equal. We might be better off removing equal.js and making fast-deep-equal an optional dependency and falling back to a more simple deep equal (like JSON.stringify or even just a one-level object comparison) if fast-deep-equal isn't included.