AddKeyToAgent adds a key to the internal ssh-key agent.

GetPasswordInput retrieves input that doesn't echo back to the user as they type it.

GetSSHAgent creates a new ssh-agent, originally it would return one only if it was needed, but it's probably better to always use our own.

LoadDefaultKey loads the default for an alias/host and puts it into the keyring if a default key is not found, the user is prompted to provide the path.

ExistingSSHAgentCallback is based on gogit's transport ssh public key callback, but allows for using an existing ssh-agent.