Add Macro-In-Macro Support (#337)

Add full macro-in-macro support so any user defined macro can contain another one as long as it was previously declared in the configuration file.

Fixes #336
Supercedes #335

proxy/config/config_windows_test.go

@@ -156,7 +156,7 @@ groups:
 		LogLevel:  "info",
 		StartPort: 5800,
 		Macros: MacroList{
-			"svr-path": "path/to/server",
+			{"svr-path", "path/to/server"},
 		},
 		Models: map[string]ModelConfig{
 			"model1": {