rename: refactor into do_rename, add rename_visual

This adds a visual mode map for renaming, which asks for the new name
via input().

`rename` itself could be changed to use `input()`, too - but I've left
it for now.

autoload/jedi.vim

@@ -179,6 +179,10 @@ function! jedi#rename(...)
     PythonJedi jedi_vim.rename()
 endfunction
 
+function! jedi#rename_visual(...)
+    PythonJedi jedi_vim.rename_visual()
+endfunction
+
 function! jedi#completions(findstart, base)
     PythonJedi jedi_vim.completions()
 endfunction