add ability to disable next/previous references with opts[key].refer === false

lib/index.js

@@ -83,8 +83,10 @@ function plugin(opts){
 
     keys.forEach(function(key){
       debug('referencing collection: %s', key);
+      var settings = opts[key];
       var col = metadata[key];
       var last = col.length - 1;
+      if (settings.refer === false) return;
       col.forEach(function(file, i){
         if (0 != i) file.previous = col[i-1];
         if (last != i) file.next = col[i+1];

test/fixtures/references-off/build/one.md

@@ -0,0 +1,2 @@
+
+one

test/fixtures/references-off/build/three.md

@@ -0,0 +1,2 @@
+
+three

test/fixtures/references-off/build/two.md

@@ -0,0 +1,2 @@
+
+two

test/fixtures/references-off/src/one.md

@@ -0,0 +1,5 @@
+---
+collection: articles
+---
+
+one

test/fixtures/references-off/src/three.md

@@ -0,0 +1,5 @@
+---
+collection: articles
+---
+
+three

test/fixtures/references-off/src/two.md

@@ -0,0 +1,5 @@
+---
+collection: articles
+---
+
+two

test/index.js

@@ -129,6 +129,23 @@ describe('metalsmith-collections', function(){
       });
   });
 
+  it('should not add references if opts[key].refer === false', function(done){
+    var metalsmith = Metalsmith('test/fixtures/references-off');
+    metalsmith
+      .use(collections({ articles: { refer: false }}))
+      .build(function(err){
+        if (err) return done(err);
+        var articles = metalsmith.metadata().articles;
+        assert(!articles[0].previous);
+        assert(!articles[0].next);
+        assert(!articles[1].previous);
+        assert(!articles[1].next);
+        assert(!articles[2].previous);
+        assert(!articles[2].next);
+        done();
+      });
+  });
+
   it('should not fail with empty collections', function(done) {
     var metalsmith = Metalsmith('test/fixtures/empty');
     metalsmith