Add ability to skip to next option if parse error

Allows providing a function to handle the case of an unknown option.

```
    public static void main(String[] args)
    {
        try
        {
            try (ConfigurationParser parser = new ConfigurationParser(new String[]{"-keep class * {}", "-unknownoption", "-whatisthisoption?"}, System.getProperties()))
            {
                parser.parse(new Configuration(), (option, location) -> {
                    System.out.println("Unknown option: " + option + " @ " + location);
                });
            }
            catch (ParseException ex)
            {
                ex.printStackTrace();
            }
        }
        catch (IOException ex)
        {
            ex.printStackTrace();
        }
    }
```

Output:

```
Unknown option: -unknownoption @ argument number 2
Unknown option: -whatisthisoption? @ argument number 3
```

base/src/main/java/proguard/ConfigurationParser.java

@@ -20,13 +20,26 @@
  */
 package proguard;
 
-import proguard.classfile.*;
+import proguard.classfile.AccessConstants;
+import proguard.classfile.ClassConstants;
+import proguard.classfile.JavaAccessConstants;
+import proguard.classfile.JavaTypeConstants;
+import proguard.classfile.TypeConstants;
 import proguard.classfile.util.ClassUtil;
-import proguard.util.*;
-
-import java.io.*;
-import java.net.*;
-import java.util.*;
+import proguard.util.ListUtil;
+import proguard.util.StringUtil;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.LineNumberReader;
+import java.io.StringReader;
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Properties;
+import java.util.function.BiConsumer;
+import java.util.function.BiFunction;
 
 
 /**
@@ -139,9 +152,24 @@ public ConfigurationParser(WordReader reader,
      * @throws IOException if an IO error occurs while reading a configuration.
      */
     public void parse(Configuration configuration)
+    throws ParseException, IOException {
+        parse(configuration, null);
+    }
+
+    /**
+     * Parses and returns the configuration.
+     *
+     * @param configuration  the configuration that is updated as a side-effect.
+     * @param unknownOptionHandler optional handler for unknown options; if null then a {@link ParseException}
+     *                             is thrown when encountering an unknown option.
+     * @throws ParseException if the any of the configuration settings contains
+     *                        a syntax error.
+     * @throws IOException    if an IO error occurs while reading a configuration.
+     */
+    public void parse(Configuration configuration, BiConsumer<String, String> unknownOptionHandler)
     throws ParseException, IOException
     {
-        while (nextWord != null)
+        parseWord: while (nextWord != null)
         {
             lastComments = reader.lastComments();
 
@@ -234,7 +262,15 @@ else if (ConfigurationConstants.REPACKAGE_CLASSES_OPTION
             else if (ConfigurationConstants.IDENTIFIER_NAME_STRING                           .startsWith(nextWord))                                                       parseUnsupportedR8Rules(ConfigurationConstants.IDENTIFIER_NAME_STRING, true);
             else
             {
-                throw new ParseException("Unknown option " + reader.locationDescription());
+                if (unknownOptionHandler != null) {
+                    unknownOptionHandler.accept(nextWord, reader.lineLocationDescription());
+                    while (nextWord != null) {
+                        readNextWord();
+                        if (nextWord != null && nextWord.startsWith("-")) {
+                            continue parseWord;
+                        }
+                    }
+                } else throw new ParseException("Unknown option " + reader.locationDescription());
             }
         }
     }