!froge command, gif support

[ArmandBriere]

Is your feature request related to a problem? Please describe.
The !froge command return a random froge, a gif can be selected and the current .png extension breaks it

Describe the solution you'd like
Change the code to identify if the selected froge emoji is a gif or a png and change the image extension
A default integration test could be added with a small gif in base64, like the image

Describe alternatives you've considered
Maybe the extension isn't required, maybe check that first