Create a struct with all of Hugo's config options

Primary motivation is documentation, but it will also hopefully simplify the code.

Also,

* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.

Closes #10896
Closes #10620

cache/filecache/integration_test.go

@@ -15,6 +15,9 @@ package filecache_test
 
 import (
 	"path/filepath"
+
+	jww "github.com/spf13/jwalterweatherman"
+
 	"testing"
 	"time"
 
@@ -62,6 +65,7 @@ title: "Home"
 -- assets/a/pixel.png --
 iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNkYPhfDwAChwGA60e6kgAAAABJRU5ErkJggg==
 -- layouts/index.html --
+{{ warnf "HOME!" }}
 {{ $img := resources.GetMatch "**.png" }}
 {{ $img = $img.Resize "3x3" }}
 {{ $img.RelPermalink }}
@@ -71,10 +75,11 @@ iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNkYPhfDwAChwGA60e6kgAA
 `
 
 	b := hugolib.NewIntegrationTestBuilder(
-		hugolib.IntegrationTestConfig{T: t, TxtarString: files, RunGC: true, NeedsOsFS: true},
+		hugolib.IntegrationTestConfig{T: t, TxtarString: files, Running: true, RunGC: true, NeedsOsFS: true, LogLevel: jww.LevelInfo},
 	).Build()
 
 	b.Assert(b.GCCount, qt.Equals, 0)
+	b.Assert(b.H, qt.IsNotNil)
 
 	imagesCacheDir := filepath.Join("_gen", "images")
 	_, err := b.H.BaseFs.ResourcesCache.Stat(imagesCacheDir)
@@ -86,9 +91,11 @@ iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNkYPhfDwAChwGA60e6kgAA
 	time.Sleep(300 * time.Millisecond)
 
 	b.RenameFile("assets/a/pixel.png", "assets/b/pixel2.png").Build()
+
 	b.Assert(b.GCCount, qt.Equals, 1)
 	// Build it again to GC the empty a dir.
 	b.Build()
+
 	_, err = b.H.BaseFs.ResourcesCache.Stat(filepath.Join(imagesCacheDir, "a"))
 	b.Assert(err, qt.Not(qt.IsNil))
 	_, err = b.H.BaseFs.ResourcesCache.Stat(imagesCacheDir)