Issue title: please update type definition for typescript user
 Issue body: Hi there,

I reference https://zimjs.com/typescript.html to start my ZIMjs project.
I find out some typing is not correct. like...

```
// zim/index.d.ts
export function zimify(obj:createjs.DisplayObject, list:boolean):DisplayObject
```

I follow this typing to write code.

```
const exportRooot: createjs.MovieClip = ...
const zContainer = zimify(exportRoot, false);
zContainer.setBounds(0,0,stageW,stageH).center();
```

It doesn't work. Because zContainer is null. And I change my code to...

```
const exportRooot: createjs.MovieClip = ...
const zContainer = zimify(exportRoot);
zContainer.setBounds(0,0,stageW,stageH).center();
```

It work well. But zimify function typing definition is not existed.

I know maintain typing definition is hard and boring.
So..Could you update ZIMjs typing definition or let me send "pull request" etc.

Thanks.

 Comments: 
 Comment 0: Hi @BibbyChung - thanks.  The second parameter should have been optional so we have updated the Typings at https://zimjs.com/typescript.html - thanks for letting us know.  And thanks for the note at https://zimjs.com/slack - it is always helpful for a quick response.  
{'source': 'GithubIssues', 'repo_name_with_owner': 'danzen/zimjs', 'issue_id': 1284455867, 'repo_stars': 359.0}