First prototype for support of custom keywords with syn's new parsing
API. Documentation and tests aren't present for now as I'm mainly
reaching for feedback.

This patch introduces a new Keyword trait, a new macro custom_keyword!
and exposes the existing TokenMarker enum. The Keyword trait
automatically implements the Token trait, making it possible to peek on
custom keywords (this is why I had to make TokenMarker public).

The custom macro generates a structure storing an Ident and implementing
the Keyword and Parse traits. A function with the same name as the
structure is also generated in order to use it like any predefined
keyword.