Speed the sanitizer up a bit, again

- allow youtube urls to start with `www`
- use `strings.Builder` instead of a `bytes.Buffer`
- use a `strings.NewReader` instead of a `bytes.NewBufferString`
- sprinkles a couple of `continue` to make the code-flow more obvious
- inline calls to `inList`, and put their parameters in the right order
- simplify isPixelTracker
- simplify `isValidIframeSource`, by extracting the hostname and comparing it
  directly, instead of using the full url and checking if it starts with
  multiple variations of the same one (`//`, `http:`, `https://` multiplied by
  ``/`www.`)
- add a benchmark

internal/reader/sanitizer/sanitizer_test.go

@@ -16,6 +16,25 @@ func TestMain(m *testing.M) {
 	os.Exit(exitCode)
 }
 
+func BenchmarkSanitize(b *testing.B) {
+	var testCases = map[string][]string{
+		"miniflux_github.html":    {"https://github.com/miniflux/v2", ""},
+		"miniflux_wikipedia.html": {"https://fr.wikipedia.org/wiki/Miniflux", ""},
+	}
+	for filename := range testCases {
+		data, err := os.ReadFile("testdata/" + filename)
+		if err != nil {
+			b.Fatalf(`Unable to read file %q: %v`, filename, err)
+		}
+		testCases[filename][1] = string(data)
+	}
+	for range b.N {
+		for _, v := range testCases {
+			Sanitize(v[0], v[1])
+		}
+	}
+}
+
 func TestValidInput(t *testing.T) {
 	input := `<p>This is a <strong>text</strong> with an image: <img src="http://example.org/" alt="Test" loading="lazy">.</p>`
 	output := Sanitize("http://example.org/", input)