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 believe the two conditions that look like this could be simplified from:

    (!opts.showPaletteOnly && typeof opts.toggleLessText == 'string') ? opts.toggleLessText : opts.toggleText

to

    opts.showPaletteOnly ? opts.toggleText : opts.toggleLessText

Since I can't think of any time where toggleLessText would not be a string.