enable cover plugin if coverage options are passed

Enable coverage plugin even if --with-coverage is not specified,
if an option to output coverage reports is passed (--cover-html,
--cover-xml) or if an option to cover files is passed (--cover-packages,
--cover-tests, --cover-inclusive)

Fixes nose-devs#514

nose/plugins/cover.py

@@ -101,6 +101,9 @@ def configure(self, options, conf):
         super(Coverage, self).configure(options, conf)
         if conf.worker:
             return
+        for opt in ["cover_packages", "cover_tests", "cover_inclusive", "cover_html", "cover_xml"]:
+            if getattr(options, opt, None) is not None:
+                self.enabled = True
         if self.enabled:
             try:
                 import coverage