Small tweak to be able to generate the invoice from a desired conf

Now you could just do

``` bash
./newinv.sh project [myproject.conf]
```

In case you have a directory of client conf files, It could be a good idea to just use that configs instead of looking for the last one in the old invoices.

I still think It could be a good idea to have the newinv functionality in the same winvo.py file, I will try to add it when have time. :-)