Rename ExternalFunction --> Plugin, and implement plugin loading.

src/org/nwapw/abacus/plugin/PluginManager.java

@@ -0,0 +1,51 @@
+package org.nwapw.abacus.plugin;
+
+import org.nwapw.abacus.number.Function;
+
+import java.lang.reflect.InvocationTargetException;
+import java.util.ArrayList;
+import java.util.HashMap;
+
+public class PluginManager {
+
+    private ArrayList<Plugin> plugins;
+    private HashMap<String, Plugin> pluginsForFunctions;
+
+    public PluginManager(){
+        plugins = new ArrayList<>();
+        pluginsForFunctions = new HashMap<>();
+    }
+
+    public Plugin pluginForFunction(String name){
+        if(pluginsForFunctions.containsKey(name)) {
+            return pluginsForFunctions.get(name);
+        }
+
+        Plugin foundPlugin = null;
+        for(Plugin plugin : plugins){
+            if(plugin.hasFunction(name)) {
+                foundPlugin = plugin;
+                break;
+            }
+        }
+        pluginsForFunctions.put(name, foundPlugin);
+
+        return foundPlugin;
+    }
+
+    public void addInstantiated(Plugin plugin){
+        plugin.load();
+        pluginsForFunctions.clear();
+        plugins.add(plugin);
+    }
+
+    public void addClass(Class<?> newClass){
+        if(!Plugin.class.isAssignableFrom(newClass)) return;
+        try {
+            addInstantiated((Plugin) newClass.getConstructor(PluginManager.class).newInstance(this));
+        } catch (InstantiationException | IllegalAccessException | NoSuchMethodException | InvocationTargetException e) {
+            e.printStackTrace();
+        }
+    }
+
+}