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()"
Duplicate URLs might occur if, for example, an artist adds another
image to his gallery while an extractor is running and images are being
downloaded on sites like pixiv/nijie/hentaifoundry.
The next image on the next page will have already been downloaded and
will cause a premature end if '--abort-on-skip' is being used.
When using 2 or more config files, the values of the second would
improperly overwrite nested dictionaries of the first one.
The new method properly combines these nested dictionaries as well.
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')
