Add rounded-corners-exclude configuration option

[elenapan]

Allows the user to selectively disable rounded corners.

Example configuration for preventing Polybar and all windows of type dock from being rounded.

rounded-corners-exclude = [
    "class_g = 'polybar'",
    "window_type = 'dock'"
];

[lwilletts]

rounded-corners-exclude = [
    "window_type = 'menu'",
    "window_type = 'tooltip'",
    "window_type = 'popup_menu'",
    "window_type = 'dropdown_menu'"
];

For those who don't want compton to affect menus.