#116 latex2e: Don't hardcode \large for subtitle

[Kirill Smelkov]

\large for document subtitle may be inappropriate, for example if main
font for title is Huge -> then \large subtitle will be looking too small.

Add ability for clients to customize.

NOTE as document subtitles are processed in depart_document(), after
self.fallbacks were processed, we emit appropriate provide command
directly.

[Günter Milde]

Thank you for the patch.
I applied an edited version (using \DUdocumentsubtitle as command name).