Adding support for predefined colour schemes

[syntaxerror]

This minor change will prevent a colorscheme setting to be written to customizer.css

Some more info about the thoughts behind this in the forum:
https://arastta.org/forum/customizer-predefined-color-schemes
https://arastta.org/forum/customizer-select-option-colour-scheme

To test, use t.ex. this in customizer.json:

    "colors": {
        "title": "text_colors_title",
        "description": "text_colors_description",
        "control": {
            "container_background-color": {
                    "type": "color",
                    "label": "text_colors_container_background_label",
                    "default": "#bdc3c7",
                    "selector": "body"
            },
            "container-color_color": {
                    "type": "color",
                    "label": "text_colors_container_color_label",
                    "default": "#666666",
                    "selector": "body"
            },
            "colorscheme": {
                "type": "select",
                "label": "text_colors_colorscheme_label",
                "default": "blue",
                "choices": {
                    "blue": "Blue",
                    "green": "Green",
                    "purple": "Purple",
                    "red": "Red"
                },
                "selector": "colors"
            }
        }
    }

[syntaxerror]

Will rethink this, trying to make it more robust for adding more settings into the Customizer ref #721