Improve a bit internal/reader/scraper/scraper.go

- make findContentUsingCustomRules' more idiomatic,
  since in golang a function returning an error might
  return garbage in other parameter. Moreover, ignoring
  errors is bad practise.
- getPredefinedScraperRules is now running in constant-time,
  instead of iterating on a list with around 50 items in it.

internal/reader/scraper/scraper_test.go

@@ -19,6 +19,10 @@ func TestGetPredefinedRules(t *testing.T) {
 		t.Error("Unable to find rule for linux.com")
 	}
 
+	if getPredefinedScraperRules("https://linux.com/") == "" {
+		t.Error("Unable to find rule for linux.com")
+	}
+
 	if getPredefinedScraperRules("https://example.org/") != "" {
 		t.Error("A rule not defined should not return anything")
 	}