Add backgroundColor to IEngineOptions

[kamranayub]

Context

You can set the backgroundColor property on an instance of engine but cannot pass it in via the constructor options.

var game = new ex.Engine({
    width: 1280,
    height: 720    
});
game.backgroundColor = ex.Color.White;

Proposal

Add backgroundColor to IEngineOptions and allow setting it that way. It should be as easy as adding a new property and then setting it in the Engine constructor.

var game = new ex.Engine({
    width: 1280,
    height: 720,
    backgroundColor: ex.Color.White
});

[htalat]

Hi,
I'd like to work on this.

[kamranayub]

Thanks @htalat! Make sure to review the contributing guidelines for what we expect and we look forward to seeing your PR!