Major improvments
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* Support auto-summation of extensions with '.ext1+.ext2' syntax. Useful for
e.g. '.c+.h'.
Minor improvements:
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* No need for --filter any longer, just pass the .exts directly as args
* Add --list flag that lists all available extensions (in the first commit)
* Rate-limit progress bar update for significant reduction in being a performance bottleneck
* Default to top-three extensions instead of complicated MIME logic
* Include git SHA1 in --version
* Stop spawning 'git log' child process, use git2-rs directly instead