Add support for "|" inside productions

Cleans up the use case where one has many small productions. For example,

@pg.production("prod: TOKEN_1")
# ... <24 lines>
@pg.production("prod: TOKEN_26")
def func(p):
pass

Would be simplified to

@pg.production("prod: TOKEN_1 | ... | TOKEN_26")
def func(p):
pass

More specifically, I need this right now, as I need to accept (and throwaway) all tokens until I see a newline.

I'd be happy to submit a PR if you guys think it's a valuable feature to have.