Use the NotFoundHandler when a file haven't been found. (#966)

This is especialy usefull when you use e.Static("/", "static") and you
want a notfoundhandler that serves your index.html like this:

echo.NotFoundHandler = func(c2 echo.Context) error {
	index := filepath.Join(c.config.StaticDir, c.config.Index)
	_, err := os.Open(index)
	if err != nil {
		return echo.ErrNotFound
	}
	return c2.File(path.Join(c.config.StaticDir, c.config.Index))
}

Another usecase with the Handler above is HTML5 SPF applications.

One caveat, you need to make sure that your NotFoundHandler doesn't
produce loops.

Signed-off-by: Rene Jochum <rene@jochums.at>

group.go

@@ -21,7 +21,7 @@ func (g *Group) Use(middleware ...MiddlewareFunc) {
 	// Allow all requests to reach the group as they might get dropped if router
 	// doesn't find a match, making none of the group middleware process.
 	g.echo.Any(path.Clean(g.prefix+"/*"), func(c Context) error {
-		return ErrNotFound
+		return NotFoundHandler(c)
 	}, g.middleware...)
 }