Give precedence ~/.ssh/config file over $SSH_AUTH_SOCK + ability set custom SSH socket path

[Dentrax]

Follow-up issue for: #26 (comment)

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By default, goph only respects SSH_AUTH_SOCK environment variable and ignores default SSH config path. This causes incorrect behavior compared to how ssh works.

case goph.HasAgent():
		auth, err = goph.UseAgent()

ssh respects ~/.ssh/config and checks if there Host * host, then uses IdentityAgent.

We should reconsider how HasAgent will work:

func HasAgent() bool {
	return os.Getenv("SSH_AUTH_SOCK") != ""
}

We could use https://github.com/kevinburke/ssh_config to check if ssh_config.Get("*", "IdentityFile") has any value. If so, give precedence to it.

Also, we should add a new arg to use custom SSH agent socket path:

func UseAgent() (Auth, error) 

This function enforces us to set $SSH_AUTH_SOCK, but we'd prefer to set optional SSH agent socket path.