Documentation generator builds index in current working directory

If you go to your nim checkout and type:

``` sh
$ nim doc --index:on lib/pure/os.nim
```

you will get a file named `os.idx` rather than `lib/pure/os.idx`. Bug or feature? I'd argue this is a bug, because the paths to the html file contained in the index file are relative to the source code and thus creating an index page from it will generate broken links.
