Add method $ to StageBase for creating ad-hoc steps using lambdas

jgiven-core/src/main/java/com/tngtech/jgiven/Stage.java

@@ -41,4 +41,5 @@ public SELF with() {
     public SELF but() {
         return self();
     }
+
 }

jgiven-core/src/main/java/com/tngtech/jgiven/StepFunction.java

@@ -0,0 +1,12 @@
+package com.tngtech.jgiven;
+
+/**
+ * A functional interface for defining ad-hoc steps.
+ *
+ * @see Stage#$(String, StepFunction)
+ * @param <STAGE> the stage in which this step is executed
+ */
+public interface StepFunction<STAGE> {
+
+    public void apply( STAGE stage ) throws Exception;
+}

jgiven-core/src/main/java/com/tngtech/jgiven/base/StageBase.java

@@ -1,5 +1,8 @@
 package com.tngtech.jgiven.base;
 
+import com.tngtech.jgiven.StepFunction;
+import com.tngtech.jgiven.annotation.Hidden;
+
 /**
  * Useful base class for step definitions as it provides a {@link #self()} method
  * to create fluent interfaces.
@@ -20,4 +23,22 @@ public SELF self() {
         return (SELF) this;
     }
 
+    /**
+     * A step method for creating ad-hoc steps using lambdas.
+     *
+     * <h2>Example Usage</h2>
+     * <pre>
+     *     given().$( "Two negative arguments", stage -> {
+     *        stage.given().argument( -5 )
+     *             .and().argument( -6 );
+     *     });
+     * </pre>
+     *
+     * @param description the description of the step
+     * @param function the implementation of the step in form of a function where the parameter is the stage the step is executed in
+     */
+    public SELF $( String description, @Hidden StepFunction<SELF> function ) throws Exception {
+        function.apply( self() );
+        return self();
+    }
 }

jgiven-tests/src/test/java/com/tngtech/jgiven/report/json/GivenJsonReports.java

@@ -52,7 +52,8 @@ public SELF the_reports_exist_as_JSON_files() throws IOException {
         return self();
     }
 
-    public void a_custom_CSS_file() throws IOException {
+    public SELF a_custom_CSS_file() throws IOException {
         customCssFile = temporaryFolderRule.newFile( "custom.css" );
+        return self();
     }
 }