Make black work like black $(pwd)

[gilramot]

Is your feature request related to a problem? Please describe.

Just writing black to the command line outputs:

Usage: black [OPTIONS] SRC ... One of 'SRC' or 'code' is required.

Describe the solution you'd like

Make the functionality of black without parameters work like black $(pwd) (excecuting black on working directory)

[denballakh]

I dont like this idea.
Black has side effects, and accidentally running it without arguments can mess up a lot of code.

[gilramot]

I dont like this idea. Black has side effects, and accidentally running it without arguments can mess up a lot of code.

Making an Are you sure? [y/N] prompt can be a good safety measure if this will ever be implemented

[JelleZijlstra]

We previously rejected this; see #740 (comment). Those reasons still apply.