Support open "current content page" in browser

This commit adds a new `--navigateToChanged` and config setting with the same name, that, when running the Hugo server with live reload enabled, will navigate to the current content file's URL on save. 

This is really useful for site-wide content changes (copyedits etc.).
Fixes #3643

commands/server.go

@@ -33,6 +33,7 @@ import (
 
 var (
 	disableLiveReload bool
+	navigateToChanged bool
 	renderToDisk      bool
 	serverAppend      bool
 	serverInterface   string
@@ -87,6 +88,7 @@ func init() {
 	serverCmd.Flags().BoolVarP(&serverWatch, "watch", "w", true, "watch filesystem for changes and recreate as needed")
 	serverCmd.Flags().BoolVarP(&serverAppend, "appendPort", "", true, "append port to baseURL")
 	serverCmd.Flags().BoolVar(&disableLiveReload, "disableLiveReload", false, "watch without enabling live browser reload on rebuild")
+	serverCmd.Flags().BoolVar(&navigateToChanged, "navigateToChanged", false, "navigate to changed content file on live browser reload")
 	serverCmd.Flags().BoolVar(&renderToDisk, "renderToDisk", false, "render to Destination path (default is render to memory & serve from there)")
 	serverCmd.Flags().String("memstats", "", "log memory usage to this file")
 	serverCmd.Flags().String("meminterval", "100ms", "interval to poll memory usage (requires --memstats), valid time units are \"ns\", \"us\" (or \"µs\"), \"ms\", \"s\", \"m\", \"h\".")
@@ -110,6 +112,10 @@ func server(cmd *cobra.Command, args []string) error {
 		c.Set("disableLiveReload", disableLiveReload)
 	}
 
+	if cmd.Flags().Changed("navigateToChanged") {
+		c.Set("navigateToChanged", navigateToChanged)
+	}
+
 	if serverWatch {
 		c.Set("watch", true)
 	}