Augtool: Add -o|--load-only option to filter loaded files

This PR adds a new `-o|--load-only` option to `augtool` which allows to choose which lens to load based on the desired files. It loads `augtool` with `--noload` and filters the lenses dynamically.

The result is faster than loading all lenses, but still definitely slower than loading with a specific lens (since it still needs to load all the lenses in order to determine which ones to keep).