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/exec.go

@@ -30,8 +30,13 @@ func (helm *execer) SetExtraArgs(args ...string) {
 	helm.extra = args
 }
 
-func (helm *execer) AddRepo(name, repository string) error {
-	out, err := helm.exec("repo", "add", name, repository)
+func (helm *execer) AddRepo(name, repository, certfile, keyfile string) error {
+	var args []string
+	args = append(args, "repo", "add", name, repository)
+	if certfile != "" && keyfile != "" {
+		args = append(args, "--cert-file", certfile, "--key-file", keyfile)
+	}
+	out, err := helm.exec(args...)
 	if helm.writer != nil {
 		helm.writer.Write(out)
 	}

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