Avoid color_disabled global state

[seaneagan]

color_disabled is global state, and since ansi color is kind of a cross-cutting concern, two different parts of an application could easily mess up each other's state. I think the best way to handle this would be to provide a means to run code in a Zone which exposes a Zone local colorDisabled value, which each ansi pen can read from.

I have a mechanism for this in the clock package:

https://github.com/seaneagan/clock/blob/master/lib/src/zoned_value.dart

Which I use for the withClock method seen here:

https://github.com/seaneagan/clock

[seaneagan]

See here for some background: chalk/chalk#46

[jtmcdole]

This package has used a global for a while. I wonder if I could store an Expando on a zone and retrieve it (since Zone.currrent[#color] cannot be set).

[jtmcdole]

I can add a Expando on Zone.current, which makes things much nicer. I'll keep color_disabled for backwards compatibility though.