hugolib: Refactor/-work the permalink/target path logic

This is a pretty fundamental change in Hugo, but absolutely needed if we should have any hope of getting "multiple outputs" done.

This commit's goal is to say:

* Every file target path is created by `createTargetPath`, i.e. one function for all.
* That function takes every page and site parameter into account, to avoid fragile string parsing to uglify etc. later on.
* The path creation logic has full test coverage.
* All permalinks, paginator URLs etc. are then built on top of that same logic.

Fixes #1252
Fixes #2110
Closes #2374
Fixes #1885
Fixes #3102
Fixes #3179
Fixes #1641
Fixes #1989

hugolib/site_output.go

@@ -14,18 +14,13 @@
 package hugolib
 
 import (
+	"path"
 	"strings"
 
+	"github.com/spf13/hugo/config"
 	"github.com/spf13/hugo/output"
 )
 
-var defaultOutputDefinitions = siteOutputDefinitions{
-	// All have HTML
-	siteOutputDefinition{ExcludedKinds: "", Outputs: []output.Type{output.HTMLType}},
-	// Some have RSS
-	siteOutputDefinition{ExcludedKinds: "page", Outputs: []output.Type{output.RSSType}},
-}
-
 type siteOutputDefinitions []siteOutputDefinition
 
 type siteOutputDefinition struct {
@@ -48,3 +43,27 @@ func (defs siteOutputDefinitions) ForKind(kind string) []output.Type {
 
 	return result
 }
+
+func createSiteOutputDefinitions(cfg config.Provider) siteOutputDefinitions {
+
+	var defs siteOutputDefinitions
+
+	// All have HTML
+	defs = append(defs, siteOutputDefinition{ExcludedKinds: "", Outputs: []output.Type{output.HTMLType}})
+
+	// TODO(bep) output deprecate rssURI
+	rssBase := cfg.GetString("rssURI")
+	if rssBase == "" {
+		rssBase = "index"
+	}
+
+	// RSS has now a well defined media type, so strip any suffix provided
+	rssBase = strings.TrimSuffix(rssBase, path.Ext(rssBase))
+	rssType := output.RSSType
+	rssType.BaseName = rssBase
+
+	// Some have RSS
+	defs = append(defs, siteOutputDefinition{ExcludedKinds: "page", Outputs: []output.Type{rssType}})
+
+	return defs
+}