Add a keyword to the syntax file

Coq allows in its grammar the use of the keyword _Remark_ as an *assertion_keyword* in the same context as _Theorem_, _Definition_, and the like, with its specific semantics: http://coq.inria.fr/refman/Reference-Manual003.html#Vernacular. I happen to use it sometimes but it's not part of the _CoqThm_ syntax group in _syntax/coq.vim_. I propose to add it with this patch.