Feature/customtarget

Add more flexible like:

```javascript
new Skin(htmlObj); // htmlObj has "data-value"
new Skin(htmlObj, {tmpl: <html string>}); // using with template
new Skin(htmlObj, {default: {name: "hoge", hp: 300, mp: 30}}); // using with default value
new Skin(htmlObj, {target: <custom data target>}); // htmlObj with "data-<custom>-value
```


and add test with mocha and chai