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)
the 'skip' config option controls skipping behavior:
true - skip download if file already exist (default)
false - download and overwrite files even if it exists
"abort" - abort extractor run if a download would be skipped
(same as '--abort-on-skip')
Mike Fährmann