Make OptFunc Public to Allow Dynamic and Conditional Options (#9)

[MuhammadSaim]

This PR addresses Issue #9 by making the optFunc type public as OptFunc. This change enables users to create and apply functional options dynamically and conditionally at runtime.

Changes Introduced

✅ Renamed optFunc → OptFunc to make it publicly accessible.
✅ Allows users to define and apply their own functional options.
✅ Ensures backward compatibility with existing option functions.
✅ Improves flexibility in configuring avatar options dynamically.

Why This Change?

Currently, users can only apply predefined options, limiting customization. With this change:

• Users can define their own option functions.
• Options can be applied conditionally at runtime based on logic.
• The functional options pattern becomes fully extensible.

Example usage

var opts []avatar.OptFunc

	// Get size from query and apply if valid
	if sizeStr := r.URL.Query().Get("size"); sizeStr != "" {
		if size, err := strconv.Atoi(sizeStr); err == nil && size > 0 {
			opts = append(opts, avatar.WithSize(size))
		}
	}

	// Get color from query and apply if valid
	if colorStr := r.URL.Query().Get("color"); colorStr != "" {
		switch colorStr {
		case "red":
			opts = append(opts, avatar.WithFgColor(color.RGBA{255, 0, 0, 255}))
		case "blue":
			opts = append(opts, avatar.WithFgColor(color.RGBA{0, 0, 255, 255}))
		case "green":
			opts = append(opts, avatar.WithFgColor(color.RGBA{0, 255, 0, 255}))
		}
	}