Update some shell config!

* New aliases:
   * 'sv' makes opening new tabs and running 'runserver' easy.
     Just new tab, 'sv', done.
   * python manage.py shell_plus saves a lot of faffing when
     creating database entries.
   * Fix the flake8 alias (I think?).
   * Add useful ack aliases for speedier searching.
* oh-my-zsh has a minor update to the Agnoster theme file.