Automatic generation of exec entries based of file exntension

[alttabber]

This allows the user to set an exec rule that would be executed automatically when a file with a specified extension is opened by Swayimg.

Ex.

[listl.autoexec]
formats = mp4,eps
mp4 = ffmpegthumbnailer -i % -o - -c png
eps = inkscape % --export-filename=- --export-type=svg

In this example, every time the user tries to open a foo.mp4 time, swayimg will automatically invoke ffmpegthumbnailer, and every time the user tries to open a foo.eps file, swayimg will automatically invoke Inkscape to convert it to svg.

This may have uses beyond just hacking in support for formats swayimg has no decoder for.

[artemsen]

The first point of Unix philosophy is:
Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new "features".

Please stop creating an all-in-one combine from an image viewer.
Image viewer should display images, that's all.
It shouldn't extract images from archives or resource files, it shouldn't convert them from one format to another, it shouldn't work directly with network or devices, it shouldn't do anything except decoding and displaying image data.

[alttabber]

This PR doesn't add any of the features you listed. All it does is adding a UI to the already existing exec:// that's in the config rather than in the CLI. Does Vim break the Unix philosophy because it's scriptable? No.

So, either exec:// breaks the Unix philosophy already, or this PR doesn't.

[artemsen]

Sorry, I still think batch processing is beyond the functionality of the image viewer and should be done by external tools.