Add option to skip cached models when downloading from command line

[chapulina]

Downloading a collection can be time consuming *, so the user may want to do it in chunks. But every new call to download a collection triggers a re-download of all its models. It would be nice to have a flag, like --skip-cached to say they should be skipped.

For example, I could start normally:

ign fuel download -j 4 -v 4 -u "https://fuel.ignitionrobotics.org/1.0/openrobotics/collections/Gazebo classic model database"

Then cancel halfway with Ctrl + C

Then restart with:

ign fuel download -j 4 -v 4 --skip-cached -u "https://fuel.ignitionrobotics.org/1.0/openrobotics/collections/Gazebo classic model database"

* or, in my case, ign-fuel-tools crashes halfway - but that's another problem