Do not require kubectl proxy: allow reading kubeconfig file

[hjacobs]

Try to read ~/.kube/config by default and use it to directly connect to the default context thus not requiring kubectl proxy anymore.

Also add --kubeconfig-path option to read arbitrary kubeconfig file containing credentials (possibly client certs #72).

[hjacobs]

Actually reading ~/.kube/config by default is impossible from within a Docker container (as user would need to mount it), so I'll keep the http://localhost:8001 proxy URL as default (this only requires passing --net=host to docker run).