SSH config support

[jbeisser]

This adds native SSH configuration support, with some minor caveats:

• It's not perfect. OpenSSH and Kevin Burke's ssh_config library behave slightly differently in compiling results. OpenSSH wants least to most specific, Burke's library wants most specific to least.
• It's ugly, but it does work. The ssh.go dialer could use some additional love.
• It also supports alternate ports in the SSH configuration file, but not very intelligently.

This DOES allow you to recycle configurations in the provider section, without setting a static username on the URI. So, if users are managed via PAM or LDAP access to the socket, it should just work.

[jbeisser]

Updated to reflect other changes in 0.7.0.

[pstrzelczak]

@jbeisser could you resolve conflicts here? @dmacvicar it is cumbersome to be forced to pass extra parameters to uri.

[dmacvicar]

thanks 🙏