Add support for specifying a custom key to be added to the trusted keys

[amiel]

This should resolve #33

Add a key with the following format:

:key:name:url

For example:

:key:nginx:https://nginx.org/keys/nginx_signing.key
:key:confluent:http://packages.confluent.io/deb/4.1/archive.key

I tested this with the following Aptfile (this is @catkhuu's example and the package I got stuck on without having a key):

:key:nginx:https://nginx.org/keys/nginx_signing.key
:repo:deb http://nginx.org/packages/ubuntu/ xenial nginx
nginx-nr-agent
:key:confluent:http://packages.confluent.io/deb/4.1/archive.key
:repo:deb [arch=amd64] http://packages.confluent.io/deb/4.0 stable main
confluent-kafka-2.11
confluent-schema-registry