Add cdxgen to list of manifest commands for tab completion (#569)

Author: pvdz

src/commands/install/socket-completion.bash

@@ -25,7 +25,7 @@ COMMANDS=(
     [install completion]=""
     [login]=""
     [logout]=""
-    [manifest]="auto conda scala gradle kotlin"
+    [manifest]="auto cdxgen conda scala gradle kotlin"
     [manifest auto]=""
     [manifest conda]=""
     [manifest cdxgen]=""