Support limited variable interpolation.

Kong supports limited variable interpolation into help strings, enum lists and
default values.

Variables are in the form:

    ${<name>}

Variables are set with the `Vars(map[string]string)` option. Undefined
variable references in the grammar will result in an error at construction
time.

interpolate_test.go

@@ -0,0 +1,17 @@
+package kong
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestInterpolate(t *testing.T) {
+	vars := map[string]string{
+		"name": "Bobby Brown",
+		"age":  "35",
+	}
+	actual, err := interpolate("${name} is ${age} years old", vars)
+	require.NoError(t, err)
+	require.Equal(t, `Bobby Brown is 35 years old`, actual)
+}