Implement CommandPermissionResolver (#135)

This pull request is an initial attempt at implementing permission for commands for the JDA module.

It includes one added file, **JDACommandPermissionResolver** which implements CommandPermissionResolver.

I created a map to store the Discord permission offsets stored mapped by their kebab-cased name (i.e. "ADMINISTRATOR" becomes "administrator", "MANAGE_SERVER" becomes "manage-server".

The implementation of `hasPermission(JDACommandEvent, String)` is fairly simple:

- Get the issuer's Member object
- Check if it's null; if so, return false.
- Get the permission offset from the aforementioned map
- Check if it's null; if so, return false.
- Return whether or not the member has the permission.

---------------------------------------

This PR allows new commands to use the `@CommandPermission` annotation like so:
```java
@CommandAlias("test|t")
public class Command extends BaseCommand {
  @Subcommand("admin|a")
  @CommandPermission("manage-server")
  public void adminCommand(MessageReceivedEvent event) {
    //...
  }
}
```

jda/src/main/java/co/aikar/commands/CommandPermissionResolver.java

@@ -2,5 +2,5 @@ package co.aikar.commands;
 
 
 public interface CommandPermissionResolver {
-    boolean hasPermission(JDACommandEvent event, String permission);
+    boolean hasPermission(JDACommandManager manager, JDACommandEvent event, String permission);
 }

jda/src/main/java/co/aikar/commands/JDACommandEvent.java

@@ -48,7 +48,7 @@ public class JDACommandEvent implements CommandIssuer {
     @Override
     public boolean hasPermission(String permission) {
         CommandPermissionResolver permissionResolver = this.manager.getPermissionResolver();
-        return permissionResolver == null || permissionResolver.hasPermission(this, permission);
+        return permissionResolver == null || permissionResolver.hasPermission(manager, this, permission);
     }
 
     @Override

jda/src/main/java/co/aikar/commands/JDACommandManager.java

@@ -91,7 +91,7 @@ public class JDACommandManager extends CommandManager<
         }
     }
 
-    private long getBotOwnerId() {
+    public long getBotOwnerId() {
         // Just in case initialization on ReadyEvent fails.
         initializeBotOwner();
         return botOwner;

jda/src/main/java/co/aikar/commands/JDACommandPermissionResolver.java

@@ -0,0 +1,34 @@
+package co.aikar.commands;
+
+import net.dv8tion.jda.core.Permission;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class JDACommandPermissionResolver implements CommandPermissionResolver {
+    private Map<String, Integer> discordPermissionOffsets;
+
+    public JDACommandPermissionResolver() {
+        discordPermissionOffsets = new HashMap<>();
+        for (Permission permission : Permission.values()) {
+            discordPermissionOffsets.put(permission.name().toLowerCase().replaceAll("_", "-"), permission.getOffset());
+        }
+    }
+
+    @Override
+    public boolean hasPermission(JDACommandManager manager, JDACommandEvent event, String permission) {
+        // Explicitly return true if the issuer is the bot's owner. They are always allowed.
+        if (manager.getBotOwnerId() == event.getIssuer().getAuthor().getIdLong()) {
+            return true;
+        }
+
+        Integer permissionOffset = discordPermissionOffsets.get(permission);
+        if (permissionOffset == null) {
+            return false;
+        }
+
+        return event.getIssuer().getMember().hasPermission(
+                Permission.getFromOffset(permissionOffset)
+        );
+    }
+}

jda/src/main/java/co/aikar/commands/JDAOptions.java

@@ -6,7 +6,7 @@ import org.jetbrains.annotations.NotNull;
 public class JDAOptions {
     CommandConfig defaultConfig = new JDACommandConfig();
     CommandConfigProvider configProvider = null;
-    CommandPermissionResolver permissionResolver = null;
+    CommandPermissionResolver permissionResolver = new JDACommandPermissionResolver();
 
     public JDAOptions() {
     }