parse error when there is spaces after `----` in slide mode

[tigercosmos]

In slide mode, we use ---- for branches.

If there are spaces after ----, the parser could not parse it as branch, but rather see it as a division line in the slide.

Is this behavior on purpose?

[SISheogorath]

At least the code base says so:

codimd/public/js/slide.js

Lines 52 to 55 in 616dfdf

	const slideOptions = {
	separator: '^(\r\n?|\n)---(\r\n?|\n)$',
	verticalSeparator: '^(\r\n?|\n)----(\r\n?|\n)$'
	}

Now let's discuss how useful/interesting it is to change it ^^

I'm fine with changing it, since ----- could still create the split line.