Changed c8 reporter html -> html-spa

- I might be the only person who uses these, but I like to dig into coverage reports so I can see where we've got holes
- In Ghost the reports is huge and unweildy, and it's hard to see what coverage is like for components grouped by folders
- html-spa reporter fixes this, it makes a beautiful nested tree view, but you can also switch to flat
- it's so much more useful for working on coverage in an area of the codebase!

.c8rc.json

@@ -2,7 +2,7 @@
   "all": true,
   "check-coverage": true,
   "reporter": [
-    "html",
+    "html-spa",
     "text-summary",
     "cobertura"
   ],