[Feature request] Allow user to exclude arbitrary directories from backup

[b02860de585071a2]

In some instances, a user may not want a particular directory (or files within this directory) to be included in a backup. It would be helpful if they were able to exclude a list of (arbitrary) directories/files.

In particular, this allows the user to avoid:

• Extremely large folders - may take up too much space on the target device, or simply take too long to be practical for regular backups
  • examples: DCIM, Music, Pictures
• Folders with sensitive contents - may cause a security concern if restored onto another device
  • examples: PGP keys, private files, work material, etc.
• Folders which contain material that is NOT desired for a later restore
  • examples: creating a "generic" backup which can be exported to other devices (regardless of manufacturer), selectively exporting data like pictures/videos for external storage (without extras like Alarms, Android, Ringtones, etc.)
• Specific files or subdirectories within a folder
  • examples: [...]/somedir/somefile.md, [...]/Documents/backup/huge_archive.tar.gz, [...]/Pictures/TOP_SECRET_BLUEPRINTS_DO_NOT_LEAK/

This would be a nice quality of life feature, and help bring this tool up to par with the functionality of other backup solutions.