Allow snippets to specify multiple markers

Example:

    @@ snip [Code.scala](../scala/Code.scala) { #imports #functions }

Fixes #21

core/src/main/scala/com/lightbend/paradox/markdown/Directive.scala

@@ -24,6 +24,8 @@ import org.pegdown.ast.DirectiveNode.Format._
 import org.pegdown.plugins.ToHtmlSerializerPlugin
 import org.pegdown.Printer
 
+import scala.collection.JavaConverters._
+
 /**
  * Serialize directives, checking the name and format against registered directives.
  */
@@ -113,9 +115,9 @@ object RefDirective {
 case class SnipDirective(page: Page) extends LeafBlockDirective("snip") {
   def render(node: DirectiveNode, visitor: Visitor, printer: Printer): Unit = {
     try {
-      val label = Option(node.attributes.identifier)
+      val labels = node.attributes.values("identifier").asScala
       val file = new File(page.file.getParentFile, node.source)
-      val text = Snippet(file, label)
+      val text = Snippet(file, labels)
       val lang = Option(node.attributes.value("type")).getOrElse(Snippet.language(file))
       new VerbatimNode(text, lang).accept(visitor)
     } catch {
@@ -142,7 +144,7 @@ object SnipDirective {
 case class FiddleDirective(page: Page) extends LeafBlockDirective("fiddle") {
   def render(node: DirectiveNode, visitor: Visitor, printer: Printer): Unit = {
     try {
-      val label = Option(node.attributes.identifier)
+      val labels = node.attributes.values("identifier").asScala
 
       val baseUrl = node.attributes.value("baseUrl", "https://embed.scalafiddle.io/embed")
       val cssClass = node.attributes.value("cssClass", "fiddle")
@@ -152,7 +154,7 @@ case class FiddleDirective(page: Page) extends LeafBlockDirective("fiddle") {
       val cssStyle = node.attributes.value("cssStyle", "overflow: hidden;")
 
       val file = new File(page.file.getParentFile, node.source)
-      val text = Snippet(file, label)
+      val text = Snippet(file, labels)
       val lang = Option(node.attributes.value("type")).getOrElse(Snippet.language(file))
 
       val fiddleSource = java.net.URLEncoder.encode(

core/src/main/scala/com/lightbend/paradox/markdown/Snippet.scala

@@ -24,9 +24,9 @@ object Snippet {
 
   class SnippetException(message: String) extends RuntimeException(message)
 
-  def apply(file: File, label: Option[String]): String = label match {
-    case Some(label) => extract(file, label)
-    case None        => extract(file, _ => true, _ => false, addFilteredLine).snippetLines.mkString("\n")
+  def apply(file: File, labels: Seq[String]): String = labels match {
+    case Seq() => extract(file, _ => true, _ => false, addFilteredLine).snippetLines.mkString("\n")
+    case _     => labels.map(label => extract(file, label)).mkString("\n")
   }
 
   def extract(file: File, label: String): String = {

plugin/src/sbt-test/paradox/snippets/expected/multiple.html

@@ -1,3 +1,6 @@
 <pre class="prettyprint"><code class="language-scala">import scala.concurrent.duration._
 
 case class Measurement(method: Method, duration: Duration)</code></pre>
+<pre class="prettyprint"><code class="language-scala">import scala.util.Try
+
+def parseInt(s: String): Option[Int] = Try(s.toInt).toOption</code></pre>

plugin/src/sbt-test/paradox/snippets/src/main/paradox/multiple.md

@@ -1 +1,3 @@
 @@ snip [multiple snippet](../../test/scala/Multiple.scala) { #multiple }
+
+@@ snip [comma separated snippet](../../test/scala/Multiple.scala) { #parseint-imports #parseint-def }

plugin/src/sbt-test/paradox/snippets/src/test/scala/Multiple.scala

@@ -1,3 +1,7 @@
+// #parseint-imports
+import scala.util.Try
+// #parseint-imports
+
 object Multiple {
   // #multiple
   import scala.concurrent.duration._
@@ -14,4 +18,9 @@ object Multiple {
   // #multiple
   case class Measurement(method: Method, duration: Duration)
   // #multiple
-}
+
+  // #parseint-def
+
+  def parseInt(s: String): Option[Int] = Try(s.toInt).toOption
+  // #parseint-def
+}