Add methods for custom number formatting in DragValue and Slider

[Adanos020]

Closes #1221.

As the title says, this PR adds a custom_formatter method for both DragValue and Slider. A custom formatter takes a f64 for the numeric value and a RangeInclusive<usize> representing the decimal range.

The following code:

ui.add(egui::Slider::new(&mut self.age, 0..=120)
    .prefix("0x")
    .custom_formatter(|n, _| format!("{:X}", n as i64)));
ui.add(egui::DragValue::new(&mut self.age)
    .custom_formatter(|n, _| format!("{:03}", n as i64)));

produces the following result:

[emilk]

Nice addition!

[Adanos020]

Applied suggested amendments and took care of the Github Actions' errors.

[emilk]

Thanks!

[Adanos020]

No worries, looking forward to the next release!