Add support for URLs relative to context root

Setting `RelativeURLs` to `true` will make all relative URLs in the site *really* relative.

And will do so with speed.

So:

In `/post/myblogpost.html`:

`/mycss.css` becomes `../mycss.css`

The same in `/index.html` will become:

`./mycss.css` etc.

Note that absolute URLs will not be touched (either external resources, or URLs constructed with `BaseURL`).

The speediness is about the same as before:

```
benchmark                    old ns/op     new ns/op     delta
BenchmarkAbsURL              17462         18164         +4.02%
BenchmarkAbsURLSrcset        18842         19632         +4.19%
BenchmarkXMLAbsURLSrcset     18643         19313         +3.59%
BenchmarkXMLAbsURL           9283          9656          +4.02%

benchmark                    old allocs     new allocs     delta
BenchmarkAbsURL              24             28             +16.67%
BenchmarkAbsURLSrcset        29             32             +10.34%
BenchmarkXMLAbsURLSrcset     27             30             +11.11%
BenchmarkXMLAbsURL           12             14             +16.67%

benchmark                    old bytes     new bytes     delta
BenchmarkAbsURL              3154          3404          +7.93%
BenchmarkAbsURLSrcset        2376          2573          +8.29%
BenchmarkXMLAbsURLSrcset     2569          2763          +7.55%
BenchmarkXMLAbsURL           1888          1998          +5.83%

```

Fixes #1104
Fixes #622
Fixes #937
Fixes #157

1 files changed, 5 insertions, 52 deletions

diff --git a/transform/absurl.go b/transform/absurl.go
index 75dbe4db3..7f06056f0 100644
--- a/transform/absurl.go
+++ b/transform/absurl.go
@@ -1,58 +1,11 @@
 package transform
 
-import (
-	"github.com/spf13/viper"
-	"sync"
-)
+var ar *absURLReplacer = newAbsURLReplacer()
 
-// to be used in tests; the live site will get its value from Viper.
-var AbsBaseUrl string
-
-var absURLInit sync.Once
-var ar *absURLReplacer
-
-func AbsURL() (trs []link, err error) {
-	initAbsURLReplacer()
-	return absURLFromReplacer(ar)
-}
-
-func absURLFromURL(URL string) (trs []link, err error) {
-	return absURLFromReplacer(newAbsURLReplacer(URL))
-}
-
-func absURLFromReplacer(ar *absURLReplacer) (trs []link, err error) {
-	trs = append(trs, func(ct contentTransformer) {
-		ar.replaceInHTML(ct)
-	})
-	return
-}
-
-func AbsURLInXML() (trs []link, err error) {
-	initAbsURLReplacer()
-	return absURLInXMLFromReplacer(ar)
-}
-
-func absURLInXMLFromURL(URL string) (trs []link, err error) {
-	return absURLInXMLFromReplacer(newAbsURLReplacer(URL))
+var AbsURL = func(ct contentTransformer) {
+	ar.replaceInHTML(ct)
 }
 
-func absURLInXMLFromReplacer(ar *absURLReplacer) (trs []link, err error) {
-	trs = append(trs, func(ct contentTransformer) {
-		ar.replaceInXML(ct)
-	})
-	return
-}
-
-func initAbsURLReplacer() {
-	absURLInit.Do(func() {
-		var url string
-
-		if AbsBaseUrl != "" {
-			url = AbsBaseUrl
-		} else {
-			url = viper.GetString("BaseURL")
-		}
-
-		ar = newAbsURLReplacer(url)
-	})
+var AbsURLInXML = func(ct contentTransformer) {
+	ar.replaceInXML(ct)
 }