RequireElements: allow slashes in element names

This lets you specify nested elements, such as specific properties,
using xpath syntax; e.g. 'properties//package-name'.

All elements are prepended with 'project//' but otherwise flexible.

Author: ctrueden

src/main/java/org/scijava/maven/plugin/enforcer/RequireElements.java

@@ -90,7 +90,7 @@ public void execute(final EnforcerRuleHelper helper)
 			fail("no elements were specified");
 		}
 		for (final String element : elements) {
-			if (element.matches("[^A-Za-z0-9_]")) {
+			if (element.matches("[^A-Za-z0-9_/]")) {
 				fail("invalid character in element name '" + element + "'");
 			}
 		}