Added option for Toggle button to show/hide the picker area.

I added an option to enable a Toggle button below the Palette area, to show or hide the picker area. This gives the user the option to have a limited number of colors to choose from, but also have the possibility to pick a different color, for advanced users.

I also updated the documentation and the example with the new options available.
I think this line could be changed to:

    if (!opts.showPaletteOnly) {
        pickerContainer.show();
    }
    else {
        pickerContainer.hide();
    }

Unless if there is some reason to use the 'left' css instead of just showing and hiding