Add filter that extracts a URL from an HTML state and turns it into a String state.

diff --git a/src/main/java/net/pterodactylus/rhynodge/filters/ExtractUrlFilter.java b/src/main/java/net/pterodactylus/rhynodge/filters/ExtractUrlFilter.java
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+++ b/src/main/java/net/pterodactylus/rhynodge/filters/ExtractUrlFilter.java
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+/*
+ * rhynodge - HttpQueryFilter.java - Copyright © 2013 David Roden
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package net.pterodactylus.rhynodge.filters;
+
+import static com.google.common.base.Preconditions.*;
+
+import net.pterodactylus.rhynodge.Filter;
+import net.pterodactylus.rhynodge.State;
+import net.pterodactylus.rhynodge.states.FailedState;
+import net.pterodactylus.rhynodge.states.HtmlState;
+import net.pterodactylus.rhynodge.states.StringState;
+
+import com.google.common.base.Optional;
+import org.jsoup.nodes.Document;
+
+/**
+ * {@link Filter} implementation that extracts a URL from an {@link HtmlState}.
+ *
+ * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+ */
+public abstract class ExtractUrlFilter implements Filter {
+
+       @Override
+       public State filter(State state) {
+               checkArgument(state instanceof HtmlState, "state must be an HTML state");
+
+               HtmlState htmlState = (HtmlState) state;
+               Optional<String> newUrl = extractUrl(htmlState.document());
+
+               if (!newUrl.isPresent()) {
+                       return new FailedState();
+               }
+
+               return new StringState(newUrl.get());
+       }
+
+       //
+       // PROTECTED METHODS
+       //
+
+       /**
+        * Extracts the URL from the given document. If the returned value is {@link
+        * Optional#absent()}, {@link #filter(State)} will return a {@link
+        * FailedState}.
+        *
+        * @param document
+        *              The document to extract the URL from
+        * @return The extracted URL, or {@link Optional#absent()}
+        */
+       protected abstract Optional<String> extractUrl(Document document);
+
+}