Add 'zt' and 'zb' to vi mode.

Author: amjith

prompt_toolkit/key_binding/bindings/vi.py

@@ -978,6 +978,7 @@ def _(event):
  return CursorRegion(pos)
 
  @handle('z', '+', filter=navigation_mode|selection_mode)
+ @handle('z', 't', filter=navigation_mode|selection_mode)
  @handle('z', Keys.ControlJ, filter=navigation_mode|selection_mode)
  def _(event):
  """
@@ -997,6 +998,7 @@ def _(event):
 
 
  @handle('z', '-', filter=navigation_mode|selection_mode)
+ @handle('z', 'b', filter=navigation_mode|selection_mode)
  def _(event):
  """
  Scrolls the window to makes the current line the last line in the visible region.