No alt tag for first letter avatar image

Hi,

The plugin does not generate the alt tag properly for the first letter avatar.

I imagine this is a quick fix for an update, but until then, I simply changes line 433 of wp-first-letter-avatar.php to include Commenter Avatar for alt=””

$output_data = "<img alt='Commenter Avatar' src='{$avatar_uri}' class='{$css_classes}' width='{$size}' height='{$size}' />";