Adding 256 color support

256 color support is added through the addition of parameterized styles

Ex:
    ansiStyles = require('ansi-styles');

    // Simple colors, existing API
    var redText = ansiStyles.red.open + 'rover' + ansiStyles.red.close;

    // 256 color example
    var eggplantText = ansiStyles.foreground(125).open + 'aubergine' + ansiStyles.foreground(125).close;

    // Also supported:
    ansiStyles.background(100);
    ansiStyles.fg(125); // alias for .foreground
    ansiStyles.bg(100); // alias for .background
    ansiStyles.fgBg(125, 100); // combines foreground and background change