Add EDITOR option in config file.

[bastien-k]

If you run sgpt in an isolated env, via pipx for example, the EDITOR variable of the host environment will not be found.

shell_gpt/sgpt/utils.py

Line 21 in 1c58566

editor = os.environ.get("EDITOR", "vim")

I suggest to have an optional EDITOR variable in the config file to cover that.