g:slime_default_config

Having a default config allows me to do something like:

```
  # in .vimrc
  let g:slime_target = "tmux"
  let g:slime_default_config = {"socket_name": "default", "target_pane": "1"}

  # in my .bashrc
  function hack-django { 
    TERM=xterm-256color
    source ./virtualenv/bin/activate
    tmux new-session "vim $*" \; splitw -v -l 10 'python ./manage.py shell'   \; select-pane -U
  }
```

As I've created the tmux in a consistent way, the pane will always be #1, so it's much nicer not to have to answer the question again each time.