Add environment variable to disable style prefix

[afbjorklund]

Some people find the "style" prefix distracting and confusing,
whether it is shown in color emoji or in 7-bit compatible text.

So add a way to opt-out from the console prefix completely, this
will just show the message text starting from the first column.

From discussion in #3724

[tstromberg]

Rather than adding another knob to support, how about improving MINIKUBE_IN_STYLE?

While I have not yet seen complaints about the ASCII mode, it could be a bit more consistent. I think it's useful to support indentation, but perhaps we should just have a single standard prefix character for info "-" and another for error.

Thoughts?

[afbjorklund]

It just seemed like a better option than filtering the output afterwards, as was their current approach...

But maybe the style output can be tweaked enough to be acceptable ? Or convert it from a boolean ?

[afbjorklund]

A fourth approach was printing the style out in full, like they do in the subtitles:

😄  minikube v1.0.0 on linux (amd64)
o   minikube v1.0.0 on linux (amd64)
minikube v1.0.0 on linux (amd64)
[happy] minikube v1.0.0 on linux (amd64)

But maybe that would work better with categories, than with the current styles.

[RA489]

/lgtm

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[tstromberg]

I'd rather not support 3 output modes if I could avoid it. Let's just tweak the default output style to be more acceptable.

Here's a straw-man proposal:

- top level item
  * second level item
! error/warning item

[afbjorklund]

I'm not going to pursue this further, so probably time to open a new issue for the original requester then.