Require space after ordered list marker

This patch fixes the issue where `0.` in the following Markdown heading is incorrectly highlighted 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