vim: Restore cursor/window position after format (#433)

Without this the cursor jumps to the top of the window after formatting
occurs.

plugin/black.vim

@@ -2,7 +2,7 @@
 " Author: Łukasz Langa
 " Created: Mon Mar 26 23:27:53 2018 -0700
 " Requires: Vim Ver7.0+
-" Version:  1.0
+" Version:  1.1
 "
 " Documentation:
 "   This plugin formats Python files.
@@ -10,6 +10,8 @@
 " History:
 "  1.0:
 "    - initial version
+"  1.1:
+"    - restore cursor/window position after formatting
 
 if v:version < 700 || !has('python3')
     echo "This script requires vim7.0+ with Python 3.6 support."
@@ -108,7 +110,9 @@ def Black():
   except Exception as exc:
     print(exc)
   else:
+    cursor = vim.current.window.cursor
     vim.current.buffer[:] = new_buffer_str.split('\n')[:-1]
+    vim.current.window.cursor = cursor
     print(f'Reformatted in {time.time() - start:.4f}s.')
 
 def BlackUpgrade():