Underscores for Horizontal Rules

[Daniel Axelrod]

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That line of underscores is being used as a horizontal rule. I've often 
seen people use underscores for horizontal rules, because most fonts 
seem to be designed so that a series of underscores appears as an 
unbroken line.

I would like to propose an addition to the Markdown syntax:
Three or more underscores create a horizontal rule the same way three 
or more asterisks or dashes are used now.

There shouldn't be too much of an ambiguity problem between that and 
emphasis, since asterisks are already used for both emphasis and 
horizontal rules.