Support reading less options from its script tag data attributes

add support to reading less script tag data attr like this:
```html
<script defer data-file-async="true"  data-verbose="false" data-log-level="1" src="js/libs/less.min.js" type="text/javascript"></script>
```
it is more easier and cleaner.

 if you found this useful, I will send an extra [commit](https://github.com/lejenome/less.js/compare/lejenome:master...addDataAttr2) that will add support to read data attributes from less files link tag too (useful it needs some specific options per less file like following)
```html
<link rel="stylesheet/less" type="text/css" href="less/styles.less" data-dump-line-numbers="all" data-relative-urls="true" data-global-vars='{ myvar: "#ddffee", mystr: "\"quoted\"" }'>
```