template/interpolate: support custom functions

template/interpolate/funcs.go

@@ -45,6 +45,11 @@ func Funcs(ctx *Context) template.FuncMap {
 	for k, v := range FuncGens {
 		result[k] = v(ctx)
 	}
+	if ctx != nil {
+		for k, v := range ctx.Funcs {
+			result[k] = v
+		}
+	}
 
 	return template.FuncMap(result)
 }

template/interpolate/i.go

@@ -11,6 +11,9 @@ type Context struct {
 	// Data is the data for the template that is available
 	Data interface{}
 
+	// Funcs are extra functions available in the template
+	Funcs map[string]interface{}
+
 	// UserVariables is the mapping of user variables that the
 	// "user" function reads from.
 	UserVariables map[string]string