Add possibility to include directory trees in config

[jeekl]

This adds another section, 'directories', to the config files, specifying a directory which to recursively traverse and add all normal files found to tarables.

The use case for this is when you want to collect several log files, that may have been rotated, for example.

[pothos]

@jeekl Thanks for sharing this. In our fork for Flatcar Container Linux we integrated your PR here but I needed a fixup in this follow-up commit to handle non-existing directories gracefully and also create the root symlinks in the tarball.

[jeekl]

@pothos Wow! Thanks, I had totally forgotten about this. :-) since this repo seems pretty abandoned, your fork seems like the one to use. Closing this.