Support reading less options from its script tag data attributes

[lejenome]

add support to reading less script tag data attr like this:

<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 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)

<link data-dump-line-numbers="all" data-relative-urls="true" data-global-vars='{ myvar: "#ddffee", mystr: "\"quoted\"" }' rel="stylesheet/less" type="text/css" href="less/styles.less">

[lukeapage]

Nice idea. Will have a think but am initially keen on it.

[jonschlinkert]

agreed, would be nice!

[lukeapage]

@seven-phases-max What do you think?

[seven-phases-max]

Assuming it's relatively lightweight, why not?

[lukeapage]

@lejenome We'll take your other commit too, please, so this also applies to less tags

[lejenome]

done

[lukeapage]

thanks then!
Would you like to make a pull request with some documentation to less-docs?

[lejenome]

I would but my English really sucks
just some notes:

• importance level: window.less < script tag < link tag
• data attributes names are not camelCase (e.g logLevel -> data-log-level)
• link tag options are just render time options (e.g verbose, logLevel, ... are not supported)
• non-string data attributes values should be JSON valid (e.g use double quotes instead of single quotes like in data-global-vars='{ myvar: "#ddffee", mystr: "\"quoted\"" }')

[lukeapage]

No problem, myself or someone else will get round to it.

[lukeapage]

docs added: less/less-docs@ebf87fc
will go live when next version released.

[kirill-konshin]

This conflicts with RequireJS's data-main attribute on line JSON.parse(tag.dataset[opt]) because tag.dataset[opt] is not JSON, it is a link to another JS file:

<script data-main="js/main.js" src="bower/requirejs/require.js" type="text/javascript"></script>

Instead of going through all options at least check for only known ones:

var stringOptions = ['env', 'dumpLineNumbers', 'rootpath', 'errorReporting',
    jsonOptions = ['...'];

// ...

if (stringOptions.indexOf(opt) != -1) {
    options[opt] = tag.dataset[opt];
} else if (jsonOptions.indexOf(opt) != -1) {
    options[opt] = JSON.parse(tag.dataset[opt]);
} else {
    // do nothing
}