Extend color support to include text attributes #65

In some environments, we cannot use color to indicate things -- for example when dealing with a monochrome monitor, or rendering for color blind folks.

It would be therefore desirable to support a SetStyle(), SetFocusedStyle(), etc. These would take just a tcell Style.

Under the hood, tcell Styles should be used, and the various APIs to set colors should just modify the style, rather than keeping separate foreground and background.

This would allow us to use underscore or reverse video or bold to indicate various things. It might allow one to use italics as well, if italics are available.

See #67 for a richer approach to this that would include theming.