Added OptionSetCustomColorCodes to pass in custom color codes.

[polymorcodeus]

Added OptionSetCustomColorCodes to pass in custom colors to color string - is ignored if not set.. Leverages colorstring.Colorize to customize progress bar to fit my 80s neon vibe. Doesn't appear as though there are any tests for ColorCodes.

Example Usage:

neonColors := map[string]string{
    "cyan":  "36",
    "pink":  "38;2;255;95;175",
    "reset": "0",
}

bar := progressbar.NewOptions(1000,
    progressbar.OptionSetWriter(ansi.NewAnsiStdout()), //you should install "github.com/k0kubun/go-ansi"
    progressbar.OptionEnableColorCodes(true),
    progressbar.OptionSetCustomColorCodes(neonColors),
    progressbar.OptionShowBytes(true),
    progressbar.OptionSetWidth(15),
    progressbar.OptionSetDescription("[cyan][1/3][reset] Writing moshable file..."),
    progressbar.OptionSetTheme(progressbar.Theme{
        Saucer:        "[pink]=[reset]",
        SaucerHead:    "[pink]>[reset]",
        SaucerPadding: " ",
        BarStart:      "[",
        BarEnd:        "]",
    }))
for i := 0; i < 1000; i++ {
    bar.Add(1)
    time.Sleep(5 * time.Millisecond)
}

Additionally:
Removed unused colorize bool from func getStringWidth as it doesn't appear to be used anymore.