Add more built-in connect options, identity, update deprecated function

[suciptoid]

Hi thank you for this library.

Here some update you can merge (or not):

1. Add more built in option (:key_cb, :user_dir) on connect option
2. Support provide private key content directly instead of file path.

identity = "---BEGIN PRIVATE KEY---...." # private key loaded from database
# or
identity = "~/.ssh/custom_rsa"

SSH.connect("localhost", identity: identity)

3. Update deprecated code & reformat mix format

For now I'm use this app directly from my git repo:

{:librarian,
       git: "https://github.com/sukacipta/librarian.git",
       ref: "47f3404f4ab061266d9fc5c53c0d231432a794f0"}