Add logic for 404 to have ugly url to build to file instead of directory

Updated RenderHomePage method with code for building 404 to a .html file instead of a directory. Fix for issue #415.
This is probably better done this way:
```go
switch targ := s.Target.(type) {
case *target.FileSystem:
	if !targ.UglyUrls {
		targ.UglyUrls = true
		defer func(){ targ.UglyUrls = false }()
	}
}
```