Default behavior of accepting and adding host keys should be flipped

[rjanja]

By default we're accepting any keys, in part to make this library more convenient to use, but to ensure security and sane defaults we should flip this behavior so that it must be explicitly set in the accept_hosts option.

[rjanja]

We should also consider renaming the accept_hosts option to silently_accept_hosts to match the corresponding :ssh.connect option.