Web component support (#211)

* Foundational components for reading/processing Wattsi files into classes.

* Introducing tests for compliance with wattsi definitions.

- Several elements have been found to be missing from the code generator.
- A long-term strategy for dealing with obsolete elements will be needed.  Should we remove and change the API?  Or should we deprecate and leave in-place?
- Found one element that was accidentally added to the code generator but never used.  Hopefully.
- Fixes will be applied in following commits.

* Added newer elements/tags and cleaned up invalid Tag class.

- Added DataTag.
- Added SlotTag and TemplateTag to support web components.
- Added global attributes to support web components.
- Removed GenerateTag.  Accidentally introduced by string replacement in code generator.

* Upgrading revapi and maven plugin. Configured revapi to ignore removal of GenerateTag.

- The GenerateTag class was created accidentally.  It does not have any representation in the HTML standard is should not be provided in this library.

code_gen/src/main/java/j2html_codegen/wattsi/AttributeDefinition.java

@@ -0,0 +1,10 @@
+package j2html_codegen.wattsi;
+
+public interface AttributeDefinition {
+
+    String name();
+
+    boolean appliesTo(ElementDefinition element);
+
+    boolean isObsolete();
+}