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.

helmexec/helmexec.go

@@ -3,7 +3,7 @@ package helmexec
 type Interface interface {
 	SetExtraArgs(args ...string)
 
-	AddRepo(name, repository string) error
+	AddRepo(name, repository, certfile, keyfile string) error
 	UpdateRepo() error
 
 	SyncRelease(name, chart string, flags ...string) error