Add `GenericDriver`, clean up Channel, system auth handling improvements and more!

• Add support for parse_genie to Response object; obviously really only for Cisco devices at this point unless there are parsers floating around out there for other platforms I don't know about!
• Add an atexit function for the ssh2 transport which forcibly closes the connection. This fixes a bug where if a user did not manually close the connection (or use a context manager for the connection) the script would hang open until an interrupt.
• Added a GenericDriver for those with non-core platforms. The GenericDriver has a really broad prompt pattern match, doesn't know about privilege levels or any other device specific stuff, but does provide the send_command , send_commands, send_interact, and get_prompt methods just like the "core" drivers do. This should be a decent starting point for anyone working on non-core platforms!
• Minor unit test improvement to cover send_commands (plural) and to cover the new GenericDriver
• Improved auth failure handling for systemssh using pty auth (username/pass auth)
• Add "failed_when" strings to the core drivers; these are used in the response object to help indicate if the channel input failed or succeeded. For scrapli not super super helpful, but nornir_scrapli will benefit from this as well!
• Modify NetworkDriver to inherit from GenericDriver -- this allowed for some clean up of how/where Response objects get created/returned from. Channel now is much more de-coupled from whatever sits on top of it (this will be important for some netconf testing happening soon!).
• Minor test de-duplication around ssh config/known hosts file gathering.
• Added a few simple examples for structured data (textfsm/genie) and updated existing examples a bit.