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.go

@@ -0,0 +1,27 @@
+package kong
+
+import (
+	"fmt"
+	"regexp"
+)
+
+var interpolationRegex = regexp.MustCompile(`(\${[[:alpha:]_][[:word:]]*})|(\$)|([^$]+)`)
+
+// Interpolate variables from vars into s for substrings in the form ${var}.
+func interpolate(s string, vars map[string]string) (string, error) {
+	out := ""
+	matches := interpolationRegex.FindAllStringSubmatch(s, -1)
+	for _, match := range matches {
+		if match[1] != "" {
+			name := match[1][2 : len(match[1])-1]
+			value, ok := vars[name]
+			if !ok {
+				return "", fmt.Errorf("undefined variable ${%s}", name)
+			}
+			out += value
+		} else {
+			out += match[0]
+		}
+	}
+	return out, nil
+}