We should check subtitle for illegal macros

[kmccurley]

The subtitle attribute of the \title macro is written to metadata, and therefore it should not contain user-defined macros. At the present time we do not check this. The title itself is allowed to contain math and accents, but not macros. We should apply the same check to subtitle. Otherwise the subtitle may not be meaningful in the captured metadata since it could contain something like \protect \LaTeX \
if the original subtitle contains the macro \LaTeX. This is but one example.