Add HelpValueFormatter as discussed in #60 (#61)

Fixes #60 .

interpolate.go

@@ -7,17 +7,6 @@ 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 := ""