Add explosions

Added explosion animation when pirates get hit. This was a quick feature that
got added as a final feature before the deadline for the Elm Slack #gamedev
Feb 2018 game jam: https://itch.io/jam/elm-game-jam-feb-2018

Elm Game Jam Feb 2018 release

First release of the game.

It was uploaded to https://itch.io/jam/elm-game-jam-feb-2018 as part of the Elm Slack #gamedev community's Feb 2018 game jam.