Allows a 16×16 flag to be displayed in a post of page using a [flag] shortcode.
Format of shortcode is [flag=gb] to display the flag of the United Kingdom of Great Britain and Northern Ireland; 247 flags are included.
Defintion of flags can be found at Wikipedia page ISO 3166-1 alpha-2: https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2