Improve markdownRule syntax matching

[psvenk]

• markdownRule now only matches if a line contains 0-3 spaces followed by the characters forming the rule (e.g. ␣␣*␣*␣*␣*␣*), with nothing else on the line. This is in accordance with CommonMark (and compatible with the original spec, which does not allow whitespace before the characters forming the rule).
• A horizontal rule can also be formed by underscores, in accordance with the original spec and CommonMark.
• An italic, bold, or bold-italic region is no longer started if the delimiter is adjacent to a copy of itself (possibly with whitespace in between). For example, an italic or bold region delimited by asterisks would not begin if the beginning asterisk(s) is/are adjacent to another asterisk. This should not have any practical consequences (italic/bold regions consisting only of whitespace would not be matched), other than that horizontal rules made of asterisks or underscores will now be matched and highlighted properly.

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[psvenk]

I apologize for the repeated notifications; I accidentally pushed another commit to this branch just now. This should now be fixed (I force-pushed HEAD^).