(feat) trim whitespace in class attributes

[dummdidumm]

fixes #339

[ptrxyz]

I think this does not shrink/remove whitespaces at the beginning of the string.
My testing pattern was simply this:

<div class="   something    ">

Trailing whitespace is removed, yet the leading one remains the same. Am I doing it wrong?

[ptrxyz]

Added comments in the code in case this was not desired behavior.

[dummdidumm]

Could you open a new issue so we can track this? Sounds like a bug - the idea was to only keep whitespace at the start of new lines after the first line.