Add support for exec based auth.

[brendandburns]

Closes #34

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[drubin]

@brendanburns Where do these files come from?

[brendandburns]

They're right there on my ....
checks notes
... my local file system.

sigh

Fixed.

[drubin]

Thanks! and thanks again for adding this @brendanburns

[brendandburns]

Fixed lint errors....

[itowlson]

/lgtm

[SawyerHopkins]

I am using aws-iam-authenticator and the response body has the shape

{ 
  "kind": "ExecCredential",
  "apiVersion": "client.authentication.k8s.io/v1alpha1",
  "spec": {},
  "status": { 
    "expirationTimestamp": "2018-12-21T23:29:01Z",
    "token": "<REDACTED>"
  }
}

Which appears correct according to https://kubernetes.io/docs/reference/access-authn-authz/authentication/

This however causes this line to return an undefined value for the bearer token. https://github.com/kubernetes-client/javascript/pull/158/files#diff-d03480d634d6f56466d9e8bf3e1b00d0R32

I have a locally linked build which returns return Bearer ${obj.status.token} which appears to work correctly.

Is this an issue with aws-iam-authenticator, is this because the auth api version is still v1alpha1, or have I setup something incorrectly in my kubectl?

users:
  - name: <REDACTED>
    user:
      auth-provider:
        name: exec
        config:
          exec:
            apiVersion: client.authentication.k8s.io/v1alpha1
            command: aws-iam-authenticator
            args:
              - token
              - -i
              - <REDACTED>
      exec:
        apiVersion: client.authentication.k8s.io/v1alpha1
        command: aws-iam-authenticator
        args:
          - token
          - -i
          - <REDACTED>

[brendandburns]

@SawyerHopkins I think this is just a straight-up bug, I didn't have a working kubeconfig to test against and I messed up....

I'll send a fix asap.

[ndeloof]

afaict this change broke the fix introduced by #132 to support electron.
shelljs isn't compatible with Electron without known possible fix yet https://github.com/shelljs/shelljs/wiki/Electron-compatibility