Require space after ordered list marker

The following would incorrectly highlight "0." in the heading as an ordered list marker:

 0.2.0   Mon Dec 3 18:27:18 2012 +0000
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 * Add missing `#ctcp_type` and `#dcc_type` accessors
 * Make `#type`, `#ctcp_type`, and `#dcc_type` return symbols, not strings