Ability to control editor URL using environment

Since the preference for editor varies by developer, not by project, it makes sense to allow the developer's environment to specify which editor better_errors links to.

Currently the `EDITOR` variable is supported, but this only matches specific, known editors that are hard-coded in better_errors.

I recommend a new environment variable, `BETTER_ERRORS_EDITOR_URL`, which would bypass inspection of `EDITOR` and override any configuration in the project. This would allow a developer to use their preferred editor without having to convince the rest of their team or modify better_errors.