added option for local build tasks

.gitignore

@@ -118,3 +118,5 @@ run/
 !gradle-wrapper.jar
 /gradlew
 /gradlew.bat
+
+local.gradle

build.gradle

@@ -46,4 +46,8 @@ shadowJar {
     relocate 'com.sparkjava', 'eu.mhsl.lib.shadow.spark-core'
 
     mergeServiceFiles()
-}
+}
+
+if (file("local.gradle").exists()) {
+    apply from: "local.gradle"
+}