No way to add a host key from a string

It seems there is currently no way to add a host key from a string, but rather only a file. See http://docs.paramiko.org/en/2.4/api/hostkeys.html.

I would suggest either exposing the HostKeys.entries attribute so that HostKeyEntrys can be added directly, or adding some proxy methods on the HostKeys class for adding/removing/updating HostKeyEntrys.

Furthermore, it would be nice if the HostKey class could load from a file-like object.