Make Page an interface

The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue  #5074, "pages from other data sources".

But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.

Most notable changes:

* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
  This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
  This improves the taxonomy template `.Title` situation and make common template constructs much simpler.

See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778

resources/resource/params.go

@@ -0,0 +1,89 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package resource
+
+import (
+	"strings"
+
+	"github.com/spf13/cast"
+)
+
+func Param(r ResourceParamsProvider, fallback map[string]interface{}, key interface{}) (interface{}, error) {
+	keyStr, err := cast.ToStringE(key)
+	if err != nil {
+		return nil, err
+	}
+
+	keyStr = strings.ToLower(keyStr)
+	result, _ := traverseDirectParams(r, fallback, keyStr)
+	if result != nil {
+		return result, nil
+	}
+
+	keySegments := strings.Split(keyStr, ".")
+	if len(keySegments) == 1 {
+		return nil, nil
+	}
+
+	return traverseNestedParams(r, fallback, keySegments)
+}
+
+func traverseDirectParams(r ResourceParamsProvider, fallback map[string]interface{}, key string) (interface{}, error) {
+	keyStr := strings.ToLower(key)
+	if val, ok := r.Params()[keyStr]; ok {
+		return val, nil
+	}
+
+	if fallback == nil {
+		return nil, nil
+	}
+
+	return fallback[keyStr], nil
+}
+
+func traverseNestedParams(r ResourceParamsProvider, fallback map[string]interface{}, keySegments []string) (interface{}, error) {
+	result := traverseParams(keySegments, r.Params())
+	if result != nil {
+		return result, nil
+	}
+
+	if fallback != nil {
+		result = traverseParams(keySegments, fallback)
+		if result != nil {
+			return result, nil
+		}
+	}
+
+	// Didn't find anything, but also no problems.
+	return nil, nil
+}
+
+func traverseParams(keys []string, m map[string]interface{}) interface{} {
+	// Shift first element off.
+	firstKey, rest := keys[0], keys[1:]
+	result := m[firstKey]
+
+	// No point in continuing here.
+	if result == nil {
+		return result
+	}
+
+	if len(rest) == 0 {
+		// That was the last key.
+		return result
+	}
+
+	// That was not the last key.
+	return traverseParams(rest, cast.ToStringMap(result))
+}