Implement Implicit "OR"

The pflang documentation says:
> If an identifier is given without a keyword, the most recent keyword is assumed. For example,
>     not host vs and ace
> is short for
>     not host vs and host ace
> which should not be confused with
>     not ( host vs or ace )

Pflua currently does not implement implicit OR, and probably should do so.  Steve McCanne seemed to think it was pretty important in http://sharkfest.wireshark.org/sharkfest.11/presentations/McCanne-Sharkfest'11_Keynote_Address.pdf, though I find it to be a source of ambiguity in the language.