Fix mapping example in the README

Strings with leading asterisks need to be escaped in YAML.

Add the same examples into the test suite to make sure they do what we
claim they do.

Fixes #102.

README.md

@@ -107,7 +107,7 @@ mappings:
     action: "$2"
     outcome: "$3"
     job: "test_dispatcher"
-- match: *.signup.*.*
+- match: "*.signup.*.*"
   labels:
     name: "signup_events_total"
     provider: "$2"

mapper_test.go

@@ -265,6 +265,41 @@ mappings:
     `,
 			configBad: true,
 		},
+		// Example from the README.
+		{
+			config: `
+mappings:
+- match: test.dispatcher.*.*.*
+  labels:
+    name: "dispatcher_events_total"
+    processor: "$1"
+    action: "$2"
+    outcome: "$3"
+    job: "test_dispatcher"
+- match: "*.signup.*.*"
+  labels:
+    name: "signup_events_total"
+    provider: "$2"
+    outcome: "$3"
+    job: "${1}_server"
+`,
+			mappings: map[string]map[string]string{
+				"test.dispatcher.FooProcessor.send.success": map[string]string{
+					"name":      "dispatcher_events_total",
+					"processor": "FooProcessor",
+					"action":    "send",
+					"outcome":   "success",
+					"job":       "test_dispatcher",
+				},
+				"foo_product.signup.facebook.failure": map[string]string{
+					"name":     "signup_events_total",
+					"provider": "facebook",
+					"outcome":  "failure",
+					"job":      "foo_product_server",
+				},
+				"test.web-server.foo.bar": map[string]string{},
+			},
+		},
 	}
 
 	mapper := metricMapper{}