Add parse method to ParserProxy

[MaxDall]

The purpose of this PR is to unify the versioning mechanics of ParserProxy and the BaseParser in a new interface called parse within ParserProxy.

#prev
extraction = parser(crawl_date).parse(html, ...)

#now
extraction = parser.parse(html, crawl_date, ...)

This change is primarily a preparation for an upcoming feature that will enable contributors to deprecate individual attributes.

Update: Maybe that's not really necessary