OpenSSH key support

[ondralukes]

Import and export functions for OpenSSH keys (generated using ssh-keygen)

[rzcoder]

Hi! Thanks for PR.
Before merging, you need to add some tests.

[ondralukes]

Hi!
I'll try to add them as soon as I can 👍

[ondralukes]

Hi,
I've added some tests and they passed.
I also tried importing SSH key, exporting it, and using it to connect to SSH server, it works, so key exporting format should be valid.

[ondralukes]

Thanks for merging 👍