Added ability to playing Gif only once, and set it as default

[AlcorSalvador]

Playing the Gif only once was a feature that is disabled in gifwrap due to spec.loops defaulting to 0 even when explicitly defined as null. omggif when it has a null value assigned to loop_count plays the gif once, 1 = twice and so forth.

[AlcorSalvador]

Understandable, Thank you for reviewing it though.

[jtlapp]

Okay, it looks good to me. Would you mind adding something to templates/README.hbs on the change in behavior, before I publish it? I'm thinking it should go in a new section near the front, maybe titled "Revisions". I'll generate the README.md from this, and it'll pull in the JSdocs.

[jtlapp]

I'm going to bump the version to 0.10.0 for your change.

[jtlapp]

Would the following do the job without breaking backward compatibility?

if (spec.loops === undefined) {
    spec.loops = 0;
}

[jtlapp]

I just noticed that there are ~67,000 repos that depend on gifwrap, so I'm really hesitant to change default behavior. I changed the solution to the one I posted above. You can pass in null to get the behavior you want.