normalize HTML case-insensitive attributes + basic support for custom elements

[rdeltour]

parse boolean/enumerated HTML attributes as case-insensitive

• add an HTMUtils facility to get whether an HTML attribute has a
  case-insensitive value (boolean and enumerated attributes)
• pre-process case-insensitive HTML attributes to lower-case their value
• refactor the pre-processing attribute logic for more clarity
• add a test

Fix #941

basic schema support for HTML custom elements

• add an isCustomElement method in HTMLUtils.
• pre-process custom elements in XMLParser to put them in the
  proprietary namespace used by the Nu Html Checker schemas to treat
  them distinctively.
• add a simple test case.

Fix #932

[mattgarrish]

Generally looks fine. The only thing I noticed is that custom element names aren't supposed to allow uppercase ascii characters, but this isn't tested. The same is true with the online validators, though.

[rdeltour]

The only thing I noticed is that custom element names aren't supposed to allow uppercase ascii characters, but this isn't tested. The same is true with the online validators, though.

Right: I applied the same (naïve) filtering as done in validator.nu. I don't know if they do some more advanced element name validation via Java assertions however.

In any case, this already gives us basic support, and I think we can safely wait for a higher integration with validator.nu to do something more advanced. As Jiminy said, I don't expect people to use custom elements in production anytime soon ;-)