Add some basic security policies with sensible defaults

This ommmit contains some security hardening measures for the Hugo build runtime.

There are some rarely used features in Hugo that would be good to have disabled by default. One example would be the "external helpers".

For `asciidoctor` and some others we use Go's `os/exec` package to start a new process.

These are a predefined set of binary names, all loaded from `PATH` and with a predefined set of arguments. Still, if you don't use `asciidoctor` in your project, you might as well have it turned off.

You can configure your own in the new `security` configuration section, but the defaults are configured to create a minimal amount of site breakage. And if that do happen, you will get clear instructions in the loa about what to do.

The default configuration is listed below. Note that almost all of these options are regular expression _whitelists_ (a string or a slice); the value `none` will block all.

```toml
[security]
  enableInlineShortcodes = false
  [security.exec]
    allow = ['^dart-sass-embedded$', '^go$', '^npx$', '^postcss$']
    osEnv = ['(?i)^(PATH|PATHEXT|APPDATA|TMP|TEMP|TERM)$']

  [security.funcs]
    getenv = ['^HUGO_']

  [security.http]
    methods = ['(?i)GET|POST']
    urls = ['.*']
```

markup/internal/external.go

@@ -2,42 +2,56 @@ package internal
 
 import (
 	"bytes"
+	"fmt"
 	"strings"
 
-	"github.com/cli/safeexec"
+	"github.com/gohugoio/hugo/common/collections"
 	"github.com/gohugoio/hugo/common/hexec"
-
 	"github.com/gohugoio/hugo/markup/converter"
 )
 
 func ExternallyRenderContent(
 	cfg converter.ProviderConfig,
 	ctx converter.DocumentContext,
-	content []byte, path string, args []string) []byte {
+	content []byte, binaryName string, args []string) ([]byte, error) {
 	logger := cfg.Logger
-	cmd, err := hexec.SafeCommand(path, args...)
-	if err != nil {
-		logger.Errorf("%s rendering %s: %v", path, ctx.DocumentName, err)
-		return nil
+
+	if strings.Contains(binaryName, "/") {
+		panic(fmt.Sprintf("should be no slash in %q", binaryName))
 	}
-	cmd.Stdin = bytes.NewReader(content)
+
+	argsv := collections.StringSliceToInterfaceSlice(args)
+
 	var out, cmderr bytes.Buffer
-	cmd.Stdout = &out
-	cmd.Stderr = &cmderr
+	argsv = append(argsv, hexec.WithStdout(&out))
+	argsv = append(argsv, hexec.WithStderr(&cmderr))
+	argsv = append(argsv, hexec.WithStdin(bytes.NewReader(content)))
+
+	cmd, err := cfg.Exec.New(binaryName, argsv...)
+	if err != nil {
+		return nil, err
+	}
+
 	err = cmd.Run()
+
 	// Most external helpers exit w/ non-zero exit code only if severe, i.e.
 	// halting errors occurred. -> log stderr output regardless of state of err
 	for _, item := range strings.Split(cmderr.String(), "\n") {
 		item := strings.TrimSpace(item)
 		if item != "" {
-			logger.Errorf("%s: %s", ctx.DocumentName, item)
+			if err == nil {
+				logger.Warnf("%s: %s", ctx.DocumentName, item)
+			} else {
+				logger.Errorf("%s: %s", ctx.DocumentName, item)
+			}
 		}
 	}
+
 	if err != nil {
-		logger.Errorf("%s rendering %s: %v", path, ctx.DocumentName, err)
+		logger.Errorf("%s rendering %s: %v", binaryName, ctx.DocumentName, err)
 	}
 
-	return normalizeExternalHelperLineFeeds(out.Bytes())
+	return normalizeExternalHelperLineFeeds(out.Bytes()), nil
 }
 
 // Strips carriage returns from third-party / external processes (useful for Windows)
@@ -45,13 +59,13 @@ func normalizeExternalHelperLineFeeds(content []byte) []byte {
 	return bytes.Replace(content, []byte("\r"), []byte(""), -1)
 }
 
-func GetPythonExecPath() string {
-	path, err := safeexec.LookPath("python")
-	if err != nil {
-		path, err = safeexec.LookPath("python.exe")
-		if err != nil {
-			return ""
+var pythonBinaryCandidates = []string{"python", "python.exe"}
+
+func GetPythonBinaryAndExecPath() (string, string) {
+	for _, p := range pythonBinaryCandidates {
+		if pth := hexec.LookPath(p); pth != "" {
+			return p, pth
 		}
 	}
-	return path
+	return "", ""
 }