Ordered list markers should be followed by a space

From the Markdown spec:

> List markers must be followed by one or more spaces or a tab.

Previously `markdown` would check for a space after unordered list markers, but not after ordered list markers - ie. "*Foo" would be (correctly) rejected by the list parser, but "1.Foo" would be (incorrectly) accepted. This patch makes `markdown` check for a space consistently.

Note that `markdown` is still not quite spec-compliant, because "*\<tab\>Foo" and "1.\<tab\>Foo" would both be rejected, when they should be accepted. However, this patch prevents the particularly annoying case where "6.5 seconds" is interpreted as a singleton list of "5 seconds".