Added a simple, unobtrusive, update checker

Closes #27

src/main/java/com/technicjelle/bluemapofflineplayermarkers/Main.java

@@ -29,6 +29,8 @@ public void onEnable() {
 
 		logger = getLogger();
 
+		UpdateChecker.check("TechnicJelle", "BlueMapOfflinePlayerMarkers", getDescription().getVersion());
+
 		getServer().getPluginManager().registerEvents(this, this);
 
 		//all actual startup and shutdown logic moved to BlueMapAPI enable/disable methods, so `/bluemap reload` also reloads this plugin
@@ -38,6 +40,7 @@ public void onEnable() {
 
 	Consumer<BlueMapAPI> onEnableListener = api -> {
 		logger.info("API Ready! BlueMap Offline Player Markers plugin enabled!");
+		UpdateChecker.logUpdateMessage(logger);
 
 		config = new Config(this);
 

src/main/java/com/technicjelle/bluemapofflineplayermarkers/UpdateChecker.java

@@ -0,0 +1,54 @@
+package com.technicjelle.bluemapofflineplayermarkers;
+
+
+import java.io.IOException;
+import java.net.HttpURLConnection;
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.util.logging.Logger;
+
+@SuppressWarnings("SameParameterValue")
+public class UpdateChecker {
+	private static boolean updateAvailable = false;
+	private static URL url = null;
+	private static String latestVersion = null;
+	private static String curVer = null;
+
+
+	static void check(String author, String name, String currentVersion) {
+		curVer = currentVersion;
+		new Thread(() -> {
+			try {
+				url = new URL("https://github.com/"+author+"/"+name+"/releases/latest");
+			} catch (MalformedURLException e) {
+				throw new RuntimeException(e);
+			}
+
+			HttpURLConnection con;
+			try {
+				con = (HttpURLConnection) url.openConnection();
+			} catch (IOException e) {
+				throw new RuntimeException(e);
+			}
+			con.setInstanceFollowRedirects(false);
+
+			String newUrl = con.getHeaderField("Location");
+
+			if(newUrl == null) {
+				throw new RuntimeException("Did not get a redirect");
+			}
+
+			String[] split = newUrl.split("/");
+			latestVersion = split[split.length - 1].replace("v", "");
+
+			if (!latestVersion.equals(curVer)) updateAvailable = true;
+		}, name + "-Update-Checker").start();
+	}
+
+	static void logUpdateMessage(Logger logger) {
+		if (updateAvailable) {
+			logger.warning("New version available: v" + latestVersion + " (current: v" + curVer + ")");
+			logger.warning("Download it at " + url);
+		}
+	}
+}