caddyhttp: Add 'map' handler (#3199)

* inital map implementation

* resolve the value during middleware execution

* use regex instead

* pr feedback

* renamed mmap to maphandler

* refactored GetString implementation

* fixed mispelling

* additional feedback

replacer.go

@@ -66,6 +66,13 @@ func (r *Replacer) Get(variable string) (interface{}, bool) {
 	return nil, false
 }
 
+// GetString  is the same as Get, but coerces the value to a
+// string representation.
+func (r *Replacer) GetString(variable string) (string, bool) {
+	s, found := r.Get(variable)
+	return toString(s), found
+}
+
 // Delete removes a variable with a static value
 // that was created using Set.
 func (r *Replacer) Delete(variable string) {