True color support

[d3nw0]

My old pull request contained bad code and failed tests. This is a better version of it that will not break old code.

[cceckman]

Suggestion (which could go in a followup PR): Rather than having a new type and a switch statement,

type Color tcell.Color
const (
  ColorYellow Color = tcell.ColorYellow
  ...
)

(and corresponding other changes).

That means that you don't have to import tcell to get the standard ~8 colors if you already have tui around, but removes a translation layer. It's also a little clearer as to where to find more information.

[marcusolsson]

The switch is a remnant from when tui supported both termbox and tcell. Now that we're only supporting tcell, I think it's safe to rewrite it like @cceckman hinted. I'd like to avoid users having to import tcell at all, and would rather mirror the color set from tcell. Type aliases might be more convenient here(?) but that would require >=go1.8.

[d3nw0]

I would suggest removing the color constant variables, and using tui.GetColor(name string) Color, which has more color coverage.

[cceckman]

Devil's advocate: Strings are not type-safe, enums are. tui.GetColor("fuschia") vs. tui.GetColor("fucshia") is a runtime error, while tui.ColorFuschia vs. tui.ColorFucshia is a compile-time error.

It would be useful to have a string-based variant - for cases where the color is a user input - but I that's not always.

[d3nw0]

Yes, but in-case the string is misspelled the default color will be returned. That way a runtime error will not exist.