split stdout & stderr auto colouring

thus for some reason if one is connected to a tty and the other a file,
we still get colour in the tty by default.

in terms of options, --color and --no-color override both, no granular
ones added since it's not worth it imo.

this is backwards compatible with custom configs setting `_COLOR`.

it could be argued that setting $_COLOR to "false" for the auto non-tty
cases is redundant, which it is, but it doesn't hurt to do so; it ensures
that if anything (inc. 3rd-party hooks and such) rely on it that it
remains correct; and ensures that if anything in the future mistakenly
uses $_COLOR instead of $_COLOR_OUT|$_COLOR_ERR that at least that will
only be broken for the use case of only one of stdout|sdterr being a tty.

Gbp-Dch: Ignore

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