Automatically add envar to help.

See #57.

interpolate.go

@@ -7,6 +7,17 @@ import (
 
 var interpolationRegex = regexp.MustCompile(`((?:\${([[:alpha:]_][[:word:]]*))(?:=([^}]+))?})|(\$)|([^$]+)`)
 
+// Returns true if the variable "v" is interpolated in "s".
+func interpolationHasVar(s string, v string) bool {
+	matches := interpolationRegex.FindAllStringSubmatch(s, -1)
+	for _, match := range matches {
+		if name := match[2]; name == v {
+			return true
+		}
+	}
+	return false
+}
+
 // Interpolate variables from vars into s for substrings in the form ${var} or ${var=default}.
 func interpolate(s string, vars map[string]string) (string, error) {
 	out := ""