Implement Implicit "OR"

[wingo]

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.