Add Support for Extended Colors

[zacharygraber]

Is your feature request related to a problem? Please describe.
The game currently runs termbox2 in its default output mode, which only offers 6 colors other than black and white. Unfortunately, there are 7 tetrominoes, meaning that either 2 pieces have to share a color (undesirable), or one of the pieces needs to be white.

Describe the solution you'd like
Adding support for additional colors, for example via termbox's 256-color mode would alleviate this problem. Not all terminals support this mode, so it may be wise to include a compile/run flag that defaults the game to its regular 6-color mode.