Enable emacs keybindings in vi insert mode (ebivim)

[mskar]

Rationale:

Emacs keybindings are the default in many UNIX shells (e.g. bash uses GNU readline) and Python consoles (including ptpython), but these bindings are unavailable when vi mode is enabled (e.g. with set -o vi in bash, bindkey -v in zsh, or repl.vi_mode = True in ptpython).

Not having access to emacs keybindings is a problem for the autosuggestions feature (repl.enable_auto_suggest = True), because the emacs bindings Ctrl-E, Ctrl-F, and Alt-F are used to accept autosuggestions.

Without these bindings, vi editing mode users will have to use the right arrow key to accept autosuggestions.
If there is one that vi users hate... it's using arrow keys.😁

There is a Condition called ebivim to turn this feature on and off. Currently, ebivim is set to True by default to facilitate testing of this feature, but it can be disabled by adding repl.emacs_bindings_in_vi_insert_mode = False to config.py.

Some users may want to disable this feature, because it conflicts with one (rather useless) vim insert mode binding: Ctrl-K(insert digraph). If anyone needs to use digraphs in ptpython, they can enter vim by pressing Ctrl-X, Ctrl-E (assuming vim is their $EDITOR) and then press Ctrl-K followed by the digraph combo (e.g. n, ~ is ñ).

Details

This pull request implements the bindings used for accepting autosuggestions, but also many others:

Please click here for the full list of keyboard shortcuts

• Alt-B: move left one word

• Alt-C: capitalize word

• Alt-D: cut right one word

• Alt-F: move right one word

• Alt-H: cut left one word

• Alt-L: lowercase word

• Alt-U: uppercase word

• Alt-Y: rotate killring

• Alt-.: insert last argument

• Ctrl-A: move to line start

• Ctrl-B: move left one character

• Ctrl-D: cut right one character

• Ctrl-E: move to line end

• Ctrl-F: move right one character

• Ctrl-H: cut left one character

• Ctrl-K: cut to line end

• Ctrl-U: cut to line start

• Ctrl-W: cut left one word

• Ctrl-Y: paste

• Ctrl-_: undo (in addition to _, - works too)

• Ctrl-X, Ctrl-E: edit in $EDITOR and execute (also works if Ctrl is released after Ctrl-X)

To install a version of ptpython with autosuggestions enabled, run:

pip install git+https://github.com/mskar/ptpython@emacs_bindings_in_vi_insert_mode