Add option to specify letters’ text attributes

[andrewsardone]

This adds the ability for a user to specify arbitrary NSAttributedString attributes to use in the letters text.

Why

My primary interest was to set the text color of the letters, but NSAttributedString attributes add a whole plethora of options.

Example

[imageView setImageWithString:@"John Smith" color:[UIColor redColor] circular:YES textAttributes:@{
    NSFontAttributeName: [UIFont fontWithName:@"Futura-Medium" size:44.0],
    NSForegroundColorAttributeName: [UIColor yellowColor],
}];

How to test

In the absence of formal unit tests, build and run the sample project and ensure nothing looks broken.