[breaking-change] Listen only on localhost by default.

It is expected that in most deployments, a reverse proxy server like
Caddy or Nginx will be connecting to Caddy; listening on any address
by default is a privacy and security concern.

src/config.go

@@ -74,9 +74,9 @@ type Config struct {
 }
 
 type ServerConfig struct {
-	Pages   string `toml:"pages" default:"tcp/:3000"`
-	Caddy   string `toml:"caddy" default:"tcp/:3001"`
-	Metrics string `toml:"metrics" default:"tcp/:3002"`
+	Pages   string `toml:"pages" default:"tcp/localhost:3000"`
+	Caddy   string `toml:"caddy" default:"tcp/localhost:3001"`
+	Metrics string `toml:"metrics" default:"tcp/localhost:3002"`
 }
 
 type WildcardConfig struct {