[Runtime] Only select UIViewController subclasses (#22)

Any example being shown by the Catalog By Convention is a UIViewController (or
subclass). Execution of Catalog By Convention code can be reduced by up to 15%
(672 ms to 570 ms in one test) by performing this filter before checking
response to the selector `catalogBreadcrumbs`.

Closes #21

Author: Robert Moore

src/private/CBCRuntime.m

@@ -94,6 +94,9 @@ BOOL CBCCatalogIsDebugLeaf(Class aClass) {
     if (hasIgnoredPrefix) {
       continue;
     }
+    if (![aClass isSubclassOfClass:[UIViewController class]]) {
+      continue;
+    }
     [classes addObject:aClass];
   }