Add support for ssh-agent

[n0v1]

It looks like protodep currently requires direct access to the private ssh key. Would it be possible to add support for ssh-agent for authentication? go-git supports this cross-platform via function NewSSHAgentAuth.

Example usage: auth_method_test.go, clone.go

[stormcat24]

@n0v1 It's possible. This proposal is a good idea, so I will implement it soon.

[hagaishapira]

@stormcat24 is there any news on this feature or a WIP? If not then I might be able to work on this sometime next week

[ksato9700]

Any news on the ssh-agent support? Is anybody already working on it?

[ewhauser]

For anyone watching this issue, please see #51 for an implementation.

[stormcat24]

@ewhauser @n0v1 @ksato9700 @hagaishapira

I'm sorry to have kept you waiting.
This feature was released as 0.1.0.
Thank you for your wonderful discussions and contributions!

[uxduck]

You could close this github issue since you've shipped the feature. Thanks, very handy feature :)