This commit mostly replaces all minus-signs ('-') in keyword names with
underscores ('_') to allow them to be used in filter-expressions. For
example 'gallery-id' got renamed to 'gallery_id'.
(It is theoretically possible to access any variable, regardless of its
name, with 'locals()["NAME"]', but that seems a bit too convoluted if
just 'NAME' could be enough)
There was never any "good" reason for the strict separation
between extractors and downloaders. This change allows for
reduced resource usage (probably unnoticeable) and less lines
of code at the "cost" of tighter coupling.
