Fix showmore markdown extension after Markdown upgrade

Markdown 2.3 moved whitespace normalization to the normalize_whitespace
preprocessor [0]. This breaks our custom showmore extension.

This commit fixes this breakage by executing the showmore preprocessor
after the normalize_whitespace preprocessor.

[0] waylan/Python-Markdown@e7e23072142eff66719fd633246957d123d78a3d