Add validation feature

[bear-lover]

Description

js-beautify makes code looks good and it has many features.
But while using it, it will be good feature that add validation feature.

I think add validate func like js_builtify or css_beautify.

[bitwiseman]

What would this function do?

[bear-lover]

its methods like js_builtify or css_beautify work well.
but if input value is not correct css format, this module works for only fomation.
To guarantee correct output, I'd like to add validation func here.
if validation return true, format func works but in other case it should not work.

[bitwiseman]

In the case of this library, if given valid input it should always produce valid output. If it doesn't that a bug.
There are other tools that can provide validation.