Add nyc test coverage output directory

[nyc](https://github.com/bcoe/nyc) is quite popular, and under some circumstances it writes to `.nyc_output` directory. This directory should be ignored by default.

Node.gitignore

@@ -14,6 +14,9 @@ lib-cov
 # Coverage directory used by tools like istanbul
 coverage
 
+# nyc test coverage
+.nyc_output
+
 # Grunt intermediate storage (http://gruntjs.com/creating-plugins#storing-task-files)
 .grunt