`pypickup update` with no packages should update all packages

[mofeing]

There should be an option to update all cached packaged. In my opinion, that should be behavior if no packages are specified in the update command.

Other option would be to have a --all flag or :all: target.

[HerManNav]

Comnand 'update' will no longer exist from release 0.4. This option could be implemented as a flag for command 'add'.

Flag -a is already in use, so it should be a different one.