Support client certs when accessing remote repos (#47)

This is important for deployments where the chart repo is protected by client cert validation.

helmfile.yaml is extended to support `certFile` and `keyFile`:

```
 repositories:
   - name: roboll
     url: http://roboll.io/charts
    certFile: optional_client_cert
    keyFile: optional_client_key
```

Everything works the same if you don't provide values for them.

Regarding the implementation, the "helm repo add" command already supports the cert-file and key-file values, so we just have to pass them through.

README.md

@@ -20,6 +20,8 @@ The default helmfile is `helmfile.yaml`:
 repositories:
   - name: roboll
     url: http://roboll.io/charts
+    certFile: optional_client_cert
+    keyFile: optional_client_key
 
 context: kube-context					 # kube-context (--kube-context)