Add Config::builder method

The intention is to reduce some confusion on how to manipulate Config.
The alternative is to make Config fields public.

Author: evanlinjin

src/config.rs

@@ -116,6 +116,10 @@ impl Config {
     pub fn validate_domain(&self) -> bool {
         self.validate_domain
     }
+
+    pub fn builder() -> ConfigBuilder {
+        ConfigBuilder::new()
+    }
 }
 
 impl Default for Config {