Cursor shape dependent on vi editing mode

[mskar]

This pull request implements vim mode-dependent changes in cursor shape.

The cursor shape is

• beam in Insert mode:
  
• block in Nav (Normal) mode
  
• underscore in Replace mode
  

The code is based on a prompt_toolkit issue.

I put the cursor code in ptpython/key_bindings.py, because the imported classes are from prompt_toolkit's key_binding module. This may be a stretch, but pressing certain keys (e.g. escape, i, a, e, R) can switch modes and change the cursor shape.

This PR only effects vi mode.

Currently, this feature is enabled by default, but it can be disabled using by adding

repl.enable_modal_cursor = False

to config.py.

To install a version of ptpython with the modal cursor, run the line below.

python -m pip install git+https://github.com/mskar/ptpython.git@vim_cursor

[jonathanslenders]

Closing in favor of: #497