Fixed SessionLoadPost event being sent at the wrong time

Greetings,

I noticed that in your plugin, the SessionLoadPost event was getting sent at the wrong time. The :mksession file's doautoall was left in place, but more commands were being appended onto the file, namely, initializing NERDTree.

I changed it so that the doautoall at the end of the :mksession file is removed, then a new one is appended onto the end of the final session file.