Add cli option / config to provide trusted gpg key IDs

[theseer]

In fully automated environments being required to interactively acknowledge the import of gpg keys is not a viable option. Instead of silently auto-importing unknown keys - which would be a security nightmare - a list of trusted key IDs should be provided.

See #66 for discussion.

[theseer]

We decided that having a configuration option in best case would be superfluous as it is equal to having the keys already imported into the keyring. Worse, it could be a security threat if specified by the phive.xml of a malicious project, auto-importing bad keys that then would automatically be trusted.

Thus, with having the cli option only, this feature would be considered implemented.