#213 Allow trimming spaces in all attributes except input value

[Simon Urli]

We recently encountered an issue because we leave the trimAttributeValues set to true, but some of our HTML forms are using leading space in some hidden inputs. So we switch this flag, but then encountered other bugs because of remaining spaces in class attribute that we would want to clean.
Here's the original issue, for more details: https://jira.xwiki.org/browse/XCOMMONS-1571

It would be great to have more fine grained control over this flag: allowing us to control it over an AttributeTransformation could be really useful in this kind of case.