calling 'abort()' in a filter aborts the current extractor run
in a cleaner way than using something like 1/0, which
causes an error message to be printed
Missing or undefined keywords will now be replaced with the value
set for 'keywords-default'. The default is Python's 'None', which
is equivalent to setting this option to JSON's 'null'.
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()"
