Use -- for endash and --- for emdash

Proper SmartyPants syntax is for double-hyphen "--" to generate an en-dash (&ndash;) and for triple-hyphen "---" to generate an em-dash (&mdash;) (see http://daringfireball.net/projects/smartypants/).

The original Discount project got this wrong, and that error was brought over to RDiscount when it was first created. Discount fixed it in https://github.com/Orc/discount/commit/40e61f6c5a4b3f9be5d9338150120a094a96a7db

This patch fixes it for RDiscount. There weren't existing tests around this, and it would be nice if this patch were accepted without them (it's really a minor change). Of course if you'd really like tests, I'd be more than happy to write some.