This allows for image filtering via Python expressions by the same
metadata that is also used to build filenames (--list-keywords).
The usually shunned eval() function is used to evaluate
filter-expressions, but it seemed quite appropriate in this case and
shouldn't introduce any new security issues, as any attacker that could do
> gallery-dl --filter "delete-everything()" ...
could as well do
> python -c "delete-everything()"
Using argument groups is a definite improvement over how things looked
previously, but general group membership of individual items might be
a thing to reconsider.
Use the '--netrc' cmdline option or set the 'netrc' config option
to 'true' to enable the use of .netrc authentication data.
The 'machine' names for the .netrc info are the lowercase extractor
names (or categories): batoto, exhentai, nijie, pixiv, seiga.
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')
