Added webResourcesValidators extension point. fixes #14

This fixes #14 

Take a look at the adopter side of this extension point here: https://github.com/gamerson/liferay-ide/commit/b6503ad4e50b015c0f0d4b16b123a4246d914da1
Yes you are right, or perhaps validate which returns a IStatus (like max suggested for HTML ignore attributes/elements)