Fix missing !exists

The !exists variable check was missing when tmux was set as the default
slime option, so it was always set to tmux even if the user set it to
"screen" in his/her vimrc.

plugin/slime.vim

@@ -53,7 +53,9 @@ endfunction
 """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
 " Public interface
 """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
-let g:slime_option = "tmux"
+if !exists("g:slime_option")
+    let g:slime_option = "tmux"
+end
 
 let s:slime_muxes = {
             \ "tmux":{"fn_config":"TmuxConfig", "fn_send":"TmuxSend", "config_var":"slime_tmux"},