Add support for draft pull requests

See https://developer.github.com/v3/pulls/#input
and https://help.github.com/en/articles/about-pull-requests#draft-pull-requests

Note that it requires the use of a github api preview: https://developer.github.com/v3/previews/#draft-pull-requests

src/main/java/org/kohsuke/github/GHPullRequest.java

@@ -52,7 +52,7 @@ public class GHPullRequest extends GHIssue {
     // details that are only available when obtained from ID
     private GHUser merged_by;
     private int review_comments, additions, commits;
-    private boolean merged, maintainer_can_modify;
+    private boolean merged, maintainer_can_modify, draft;
     private Boolean mergeable;
     private int deletions;
     private String mergeable_state;
@@ -187,6 +187,11 @@ public class GHPullRequest extends GHIssue {
         return maintainer_can_modify;
     }
 
+    public boolean isDraft() throws IOException {
+        populate();
+        return draft;
+    }
+
     /**
      * Is this PR mergeable?
      *