twitch.go: fix Videos decl of a couple of fields that are sometimes null
refetch cached data (Season 2 Ro16 is done)
rm vodpile-demodata.js and references thereof

Not needed anymore; we have a full cache of all archived video metadata
from the channel (well, at least, all the data parsed by our golang
code; this omits some fields).
rename hierarchy field in TITLE_FORMATS to matchFields
add twitch/dump/dump.go and rule for making a title dump

Will make it easier for me to play around with title regexes.
update title parsing to fill fields using constants/defaults as needed

+ Add global default descriptor values.
+ Add in per-title-format descriptor values in the 'constants' field of
  the title format.
+ Reintroduce hierarchies, but make them separate from the "match"
  fields" which are extracted from a title.  Descriptors now explicitly
  refer to the hierarchy to which they belong.
+ Sanity check descriptors after parsing, and log on error.