support fish vi mode and other on-variable changes

[soraxas]

This PR closes #34 and #45 where it will repaint prompt when variable changes.

There's a variable $async_prompt_on_variable that controls which variable would fire the repaint function (by default it includes fish_bind_mode).

To fix #45, one can set

set -U async_prompt_on_variable fish_bind_mode PWD

to execute the async prompt when $PWD changes (I've tested and it does works).

NOTE: I wasn't sure why #34 mentioned that --on-variable does not work. It works when I'm testing on my fish.

$ fish --version
fish, version 3.1.2

[soraxas]

The switch case is needed because fish_bind_mode gets filtered out on the next case.

[acomagu]

Thank you!