Add config.cascade

This commit adds support for using the `cascade` keyword in your configuration file(s), e.g. `config.toml`.

Note that

* Every feature of `cascade` is available, e.g. `_target` to target specific page sets.
* Pages, e.g. the home page, can overwrite the cascade defined in config.

Fixes #8741

resources/page/page_matcher.go

@@ -19,6 +19,7 @@ import (
 
 	"github.com/pkg/errors"
 
+	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/hugofs/glob"
 	"github.com/mitchellh/mapstructure"
 )
@@ -70,6 +71,42 @@ func (m PageMatcher) Matches(p Page) bool {
 	return true
 }
 
+// DecodeCascade decodes in which could be eiter a map or a slice of maps.
+func DecodeCascade(in interface{}) (map[PageMatcher]maps.Params, error) {
+	m, err := maps.ToSliceStringMap(in)
+	if err != nil {
+		return map[PageMatcher]maps.Params{
+			{}: maps.ToStringMap(in),
+		}, nil
+	}
+
+	cascade := make(map[PageMatcher]maps.Params)
+
+	for _, vv := range m {
+		var m PageMatcher
+		if mv, found := vv["_target"]; found {
+			err := DecodePageMatcher(mv, &m)
+			if err != nil {
+				return nil, err
+			}
+		}
+		c, found := cascade[m]
+		if found {
+			// Merge
+			for k, v := range vv {
+				if _, found := c[k]; !found {
+					c[k] = v
+				}
+			}
+		} else {
+			cascade[m] = vv
+		}
+	}
+
+	return cascade, nil
+
+}
+
 // DecodePageMatcher decodes m into v.
 func DecodePageMatcher(m interface{}, v *PageMatcher) error {
 	if err := mapstructure.WeakDecode(m, v); err != nil {