Add a Codecov configuration file to adjust the defaults.

In particular, we're not 100% sold on the Codecov charts - which add some
noise, but seem like they could be helpful at higher coverage percentages.

Also, for the time-being, we'll lower the target for `patch`s (i.e. PRs) to
zero, just to see how the values get reported in the CI checks.

Ref: https://docs.codecov.io/docs/commit-status

codecov.yml

@@ -0,0 +1,13 @@
+coverage:
+  status:
+    patch:
+      default:
+        # We should raise this, but we want to evaluate how
+        # this looks in practice, first.  We can do that for now
+        # by observing the status checks on the PRs themselves.
+        target: 0%
+
+# The charts can still be viewed by clicking on the status
+# checks, but they create a fair amount of noise by default.
+comment: off
+