Feature: Image Selection / Filtering

[mikf]

Selecting only a certain type/volume/etc of manga chapters has been asked about a couple of times (#25, #11 (comment)) and using metadata, which is usually only used to build filenames, to select/filter images or image-groups seems like a generally useful feature.

The foundation for this has already been put into place with 9b21d3f and 0dedbe7, which add the --filter and --chapter-filter cmdline options. These allow users to specify their own Python expressions to filter out images for which the expression evaluates to False. The available variable/keyword names for a specific URL can be looked up using -K or -j.

For example, to only download chapter 3 to 5 in English:
gallery-dl --chapter-filter "lang == 'en' and 3 <= chapter <= 5" https://bato.to/comic/_/comics/fury-r12548

These user-specified Python expression are currently evaluated using the builtin eval function, which is generally considered unsafe for input from untrusted sources (Eval really is dangerous), but that shouldn't be an issue for this particular use-case (unless someone has a good argument against using it). On the plus side: eval is a lot faster and more powerful then for example youtube-dl's --match-filter, but there are probably good reasons why youtube-dl doesn't use eval for their filter implementation.

Todo:

• Implement --filter and --chapter-filter
• Provide manga-chapter metadata
• Decide on sensible globals for filter expressions