Allow better linenum override in CodeHilite

Fixes #148. The "force_linenos" config setting of the CodeHilite extension has been
marked as Pending Deprecation and a new setting "linenums" has been added to
replace it. See documentation for the [CodeHilite Extension] for an explaination
of the new "linenums" setting. The new setting will honor the old "force_linenos"
if it is set, but it will raise a PendingDeprecationWarning and will likely be
removed in a future version of Python-Markdown.

[CodeHilite Extension]: extensions/codehilite.html