Add ExSync tool to "make relive" with Mix

How to use:
- Compile ejabberd with Mix
- Start ejabberd with "make relive"
- Edit some ejabberd source code file
- Save the file, and ExSync will compile and reload it automatically

ExSync depends on FileSystem library, which requires inotify-tools, see
https://github.com/falood/file_system#system-support

References:
https://hex.pm/packages/exsync
https://github.com/falood/exsync

config/runtime.exs

@@ -11,3 +11,5 @@ config :ejabberd,
   log_path: Path.join(rootpath, "logs/ejabberd.log")
 config :mnesia,
   dir: Path.join(rootpath, "database/")
+config :exsync,
+  reload_callback: {:ejabberd_admin, :update, []}