Implement XML data support

Example:

```
{{ with resources.Get "https://example.com/rss.xml" | transform.Unmarshal }}
    {{ range .channel.item }}
        <strong>{{ .title | plainify | htmlUnescape }}</strong><br />
        <p>{{ .description | plainify | htmlUnescape }}</p>
        {{ $link := .link | plainify | htmlUnescape }}
        <a href="{{ $link }}">{{ $link }}</a><br />
        <hr>
    {{ end }}
{{ end }}
```

Closes #4470

1 files changed, 9 insertions, 0 deletions

diff --git a/parser/frontmatter.go b/parser/frontmatter.go
index 79701a0fc..a99829521 100644
--- a/parser/frontmatter.go
+++ b/parser/frontmatter.go
@@ -23,6 +23,8 @@ import (
 	toml "github.com/pelletier/go-toml/v2"
 
 	yaml "gopkg.in/yaml.v2"
+
+	xml "github.com/clbanning/mxj/v2"
 )
 
 const (
@@ -62,7 +64,14 @@ func InterfaceToConfig(in interface{}, format metadecoders.Format, w io.Writer)
 
 		_, err = w.Write([]byte{'\n'})
 		return err
+	case metadecoders.XML:
+		b, err := xml.AnyXmlIndent(in, "", "\t", "root")
+		if err != nil {
+			return err
+		}
 
+		_, err = w.Write(b)
+		return err
 	default:
 		return errors.New("unsupported Format provided")
 	}