Added the ability to change Checkbox icon

[casperstorm]

This PR extends Checkbox so its now possible to customize the icon.
The Icon is defined by the following.

pub struct Icon<Font> {
    pub font: Font,
    pub code_point: char,
    pub size: Option<u16>,
}

I renamed checkmark_color to icon_color inside checkbox::Appearance since it feels more fitting with a custom icon that might not be a checkmark.

To test run cargo run --package checkbox

[hecrj]

Looks awesome! Thanks! 🎉