[scrapers] Replace Buffers before parsing XSLT nodes

Had a chance to play around a little with the new XSLT scraping with the tvdb scraper, but one thing I noticed was that I was unable to refer to any buffers or settings within the XSLT code.  

This makes it pretty difficult, for example, to refer to a specific episode in GetEpisodeDetails, where we need the episode id stored in $$2 to determine which Episode node to parse.  Or for grabbing posters, where we want to prioritize posters which match $INFO[language].

I can't see any specific harm in replacing the buffers - $foo in XSLT refers to a variable, so at worst it only makes $INFO and $LOCALIZE "reserved" terms, and scraper developers would be free to pick a different variable name to use.  I don't think $$1-$$20 have any special meaning either.

(Ideally for Gotham... It's, uh, a "fix" for an already added feature...)

@night199uk and @jmarshallnz