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

1 files changed, 57 insertions, 0 deletions

diff --git a/resources/page/page_data_test.go b/resources/page/page_data_test.go
new file mode 100644
index 000000000..b6641bcd7
--- /dev/null
+++ b/resources/page/page_data_test.go
@@ -0,0 +1,57 @@
+// 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 page
+
+import (
+	"bytes"
+	"testing"
+
+	"text/template"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestPageData(t *testing.T) {
+	assert := require.New(t)
+
+	data := make(Data)
+
+	assert.Nil(data.Pages())
+
+	pages := Pages{
+		&testPage{title: "a1"},
+		&testPage{title: "a2"},
+	}
+
+	data["pages"] = pages
+
+	assert.Equal(pages, data.Pages())
+
+	data["pages"] = func() Pages {
+		return pages
+	}
+
+	assert.Equal(pages, data.Pages())
+
+	templ, err := template.New("").Parse(`Pages: {{ .Pages }}`)
+
+	assert.NoError(err)
+
+	var buff bytes.Buffer
+
+	assert.NoError(templ.Execute(&buff, data))
+
+	assert.Contains(buff.String(), "Pages(2)")
+
+}