Various improvements to plugin metadata

- Add plugin (display) name and plugin URL
 - Make everything except plugin ID optional (instead of empty string)
 - Exclude empty properties from generated velocity-plugin.json
 - Make plugin author list immutable
 - Other (minor) cleanup and refactoring

.gitignore

@@ -130,4 +130,5 @@ gradle-app.setting
 logs/
 /velocity.toml
 server-icon.png
-/bin/
+/bin/
+run/