add cited-only tests

features/cited_only.feature

+Feature: Cited-only Bibliographies
+  As a scholar who likes to blog
+  I want to cite references on my website
+  And generate bibliographies for the cited items
+
+  Scenario: Cited-only references from a single bibliography
+    Given I have a scholar configuration with:
+      | key    | value             |
+      | source | ./_bibliography   |
+    And I have a "_bibliography" directory
+    And I have a file "_bibliography/references.bib":
+      """
+      @book{ruby,
+        title     = {The Ruby Programming Language},
+        author    = {Flanagan, David and Matsumoto, Yukihiro},
+        year      = {2008},
+        publisher = {O'Reilly Media}
+      },
+      @book{smalltalk,
+        title     = {Smalltalk Best Practice Patterns},
+        author    = {Kent Beck},
+        year      = {1996},
+        publisher = {Prentice Hall}
+      }
+
+      """
+    And I have a page "scholar.html":
+      """
+      ---
+      ---
+      {% cite smalltalk %}
+      {% bibliography --cited %}
+      """
+    When I run jekyll
+    Then the _site directory should exist
+    And the "_site/scholar.html" file should exist
+    And I should not see "<i>The Ruby Programming Language</i>" in "_site/scholar.html"
+    And I should see "<i>Smalltalk Best Practice Patterns</i>" in "_site/scholar.html"
+
+  Scenario: No-cited items result in empty bibliography
+    Given I have a scholar configuration with:
+      | key    | value             |
+      | source | ./_bibliography   |
+    And I have a "_bibliography" directory
+    And I have a file "_bibliography/references.bib":
+      """
+      @book{ruby,
+        title     = {The Ruby Programming Language},
+        author    = {Flanagan, David and Matsumoto, Yukihiro},
+        year      = {2008},
+        publisher = {O'Reilly Media}
+      },
+      @book{smalltalk,
+        title     = {Smalltalk Best Practice Patterns},
+        author    = {Kent Beck},
+        year      = {1996},
+        publisher = {Prentice Hall}
+      }
+
+      """
+    And I have a page "scholar.html":
+      """
+      ---
+      ---
+      {% bibliography --cited %}
+      """
+    When I run jekyll
+    Then the _site directory should exist
+    And the "_site/scholar.html" file should exist
+    And I should not see "<i>The Ruby Programming Language</i>" in "_site/scholar.html"
+    And I should not see "<i>Smalltalk Best Practice Patterns</i>" in "_site/scholar.html"
+