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

helpers/path.go

@@ -232,6 +232,41 @@ func MakePathRelative(inPath string, possibleDirectories ...string) (string, err
 	return inPath, errors.New("Can't extract relative path, unknown prefix")
 }
 
+// Should be good enough for Hugo.
+var isFileRe = regexp.MustCompile(".*\\..{1,6}$")
+
+// Expects a relative path starting after the content directory.
+func GetDottedRelativePath(inPath string) string {
+	inPath = filepath.Clean(filepath.FromSlash(inPath))
+	if inPath == "." {
+		return "./"
+	}
+	isFile := isFileRe.MatchString(inPath)
+	if !isFile {
+		if !strings.HasSuffix(inPath, FilePathSeparator) {
+			inPath += FilePathSeparator
+		}
+	}
+	if !strings.HasPrefix(inPath, FilePathSeparator) {
+		inPath = FilePathSeparator + inPath
+	}
+	dir, _ := filepath.Split(inPath)
+
+	sectionCount := strings.Count(dir, FilePathSeparator)
+
+	if sectionCount == 0 || dir == FilePathSeparator {
+		return "./"
+	}
+
+	var dottedPath string
+
+	for i := 1; i < sectionCount; i++ {
+		dottedPath += "../"
+	}
+
+	return dottedPath
+}
+
 // Filename takes a path, strips out the extension,
 // and returns the name of the file.
 func Filename(in string) (name string) {