Add helpful text for `standard --format`

[junosuarez]

Issue extracted from conversation at #340

As of standard@6, standard-format is no longer included in the standard package. This is a feature request to add helpful forwarding text instructing people to instead download standard-format if they try running standard --format to ensure a smoother upgrade experience.

[feross]

This is already the case with standard v6.

$ standard --format
standard: Formatting is no longer included with standard. Install it separately: "npm install -g standard-format"

Is it not for you?

[junosuarez]

Ah, it is, my path was picking up an older standard bin.

[feross]

@jden Excellent :)