Added ability to set context in charts.yaml (#21)

* Added ability to set context in charts.yaml

* Added context to the documentation
Added error if —kube-context and context are both used.

main.go

@@ -225,7 +225,13 @@ func before(c *cli.Context) (*state.HelmState, helmexec.Interface, error) {
 	if err != nil {
 		return nil, nil, err
 	}
-
+	if state.Context != "" {
+		if kubeContext != "" {
+			log.Printf("err: Cannot use option --kube-context and set attribute context.")
+			os.Exit(1)
+		}
+		kubeContext = state.Context
+	}
 	var writer io.Writer
 	if !quiet {
 		writer = os.Stdout