Added a mechanism to filter extension points as they are discovered.

I couldn't quite find one place to do this, so it's spread apart
somewhat.

core/src/main/java/hudson/ExtensionFinder.java

@@ -41,6 +41,7 @@ import hudson.init.InitMilestone;
 import hudson.model.Descriptor;
 import hudson.model.Hudson;
 import jenkins.ExtensionComponentSet;
+import jenkins.ExtensionFilter;
 import jenkins.ExtensionRefreshException;
 import jenkins.ProxyInjector;
 import jenkins.model.Jenkins;
@@ -72,10 +73,11 @@ import java.lang.reflect.Method;
  *
  * <p>
  * {@link ExtensionFinder} itself is an extension point, but to avoid infinite recursion,
- * Hudson discovers {@link ExtensionFinder}s through {@link Sezpoz} and that alone.
+ * Jenkins discovers {@link ExtensionFinder}s through {@link Sezpoz} and that alone.
  *
  * @author Kohsuke Kawaguchi
  * @since 1.286
+ * @see ExtensionFilter
  */
 public abstract class ExtensionFinder implements ExtensionPoint {
     /**
@@ -126,7 +128,7 @@ public abstract class ExtensionFinder implements ExtensionPoint {
      * <p>
      * This method should return all the known components at the time of the call, including
      * those that are discovered later via {@link #refresh()}, even though those components
-     * are separately retruend in {@link ExtensionComponentSet}.
+     * are separately returned in {@link ExtensionComponentSet}.
      *
      * @param <T>
      *      The type of the extension points. This is not bound to {@link ExtensionPoint} because