Our tcell library incorrectly referenced tcell color values rather than
just names. The values aren't part of the public API and did change with
the update to v2.
This commit switches our tcell library to using the public constants
exported by the `tcell` project.
Also aligning our color definition of the first 16 colors with `tcell`
and Xterm. Adding two additional colors to make this change backward
compatible with `termbox-go`.
