Validate that template's markup is valid

[gforcada]

There was a template with broken HTML, we should know that automatically, so we can fix that whenever that happens, i.e. a pre-commit hook 🎉

@petschki found out this: https://github.com/Lucas-C/pre-commit-hooks-lxml/tree/master/pre_commit_hooks we could create a pre-commit hook here in plone/meta or somewhere else in collective, the location is not that important, to validate that our template files are actually valid 💯

[petschki]

found this one yesterday: https://github.com/Lucas-C/pre-commit-hooks-java/

though it says "needs java in $PATH" the id validate-html could be used 1:1 for that.