Added breaking change count to core design

Everything will now calculate total changes and breaking changes as a convenience and aggregation mechanism.

Signed-off-by: Dave Shanley <dave@quobix.com>

what-changed/external_docs_test.go

@@ -115,6 +115,7 @@ url: https://quobix.com`
 	assert.True(t, extChange.Context.HasChanged())
 	assert.Equal(t, "hiya!", extChange.New)
 	assert.Equal(t, "hello", extChange.Original)
+	assert.Equal(t, 0, extChanges.TotalBreakingChanges())
 }
 
 func TestCompareExternalDocs_Identical(t *testing.T) {