Add an "assets_dir" option for frpc

This option allows users to specify where they want assets to be loaded
from, like the "assets_dir" option that already exists for frps. This
allows library users to use the admin panel without having to bundle
assets with statik.

conf/frpc_full.ini

@@ -29,6 +29,8 @@ admin_addr = 127.0.0.1
 admin_port = 7400
 admin_user = admin
 admin_pwd = admin
+# Admin assets directory. By default, these assets are bundled with frpc.
+# assets_dir = ./static
 
 # connections will be established in advance, default value is zero
 pool_count = 5