conditional for EPUB?

Hello,

I would like to propose a conditional for EPUB, in case a different
Texinfo code is needed than for regular HTML.  A first use case is the
metadata proposal, the EPUB and HTML case need to be different.  There
could be other uses, for instance related to differences in the title
page.

I do not think that an additional raw format block command is needed,
@html can be used for that purpose.

The command line is --epub3, but I would propose @ifepub for the
conditional.

Thoughts?

-- 
Pat