Image resource refactor

This commit pulls most of the image related logic into its own package, to make it easier to reason about and extend.

This is also a rewrite of the transformation logic used in Hugo Pipes, mostly to allow constructs like the one below:

    {{ ($myimg | fingerprint ).Width }}

Fixes #5903
Fixes #6234
Fixes #6266

resources/resource_transformers/minifier/minify_test.go

@@ -0,0 +1,43 @@
+// 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 minifier
+
+import (
+	"testing"
+
+	"github.com/gohugoio/hugo/resources/resource"
+
+	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/resources/resource_transformers/htesting"
+)
+
+func TestTransform(t *testing.T) {
+	c := qt.New(t)
+
+	spec, err := htesting.NewTestResourceSpec()
+	c.Assert(err, qt.IsNil)
+	client := New(spec)
+
+	r, err := htesting.NewResourceTransformerForSpec(spec, "hugo.html", "<h1>   Hugo Rocks!   </h1>")
+	c.Assert(err, qt.IsNil)
+
+	transformed, err := client.Minify(r)
+	c.Assert(err, qt.IsNil)
+
+	c.Assert(transformed.RelPermalink(), qt.Equals, "/hugo.min.html")
+	content, err := transformed.(resource.ContentProvider).Content()
+	c.Assert(err, qt.IsNil)
+	c.Assert(content, qt.Equals, "<h1>Hugo Rocks!</h1>")
+
+}