Handler: add SetIndex

This allows setting an index.html (or equivalent) filename; this means that
we can serve requests to "/foo" with the contents of "/foo/index.html".

handler.go

@@ -5,6 +5,7 @@ import (
 	"net/http"
 	"os"
 	"path"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"time"
@@ -85,6 +86,28 @@ func (h *Handler) SetHeader(key, value string) {
 	}
 }
 
+// SetIndex allows setting an index.html (or equivalent) that can be used to
+// serve requests landing at a directory. For instance, if a file named
+// "/foo/index.html" exists, and this function is called with "index.html",
+// then a route will be registered to serve the contents of this file at
+// "/foo". Noting that the ServeHTTP handler discards a trailing "/" on non
+// root URLs, this means that it will serve equivalent content for requests
+// to "/foo/index.html", "/foo/" and "/foo".
+//
+// Existing routes are not overwritten, and this function could be called
+// multiple times with different filenames (noting later calls would not
+// overwrite files matching earlier calls).
+func (h *Handler) SetIndex(filename string) {
+	for k, v := range h.dir {
+		if filepath.Base(k) == filename {
+			routeToAdd := filepath.Dir(k)
+			if _, exists := h.dir[routeToAdd]; !exists {
+				h.dir[routeToAdd] = v
+			}
+		}
+	}
+}
+
 // ServeHTTP handles requests for files. It supports GET and HEAD methods, with
 // anything else returning a 405. Exact path matches are required, else a 404 is
 // returned.