make-initial-stoptags looks for ".ht" extension but load-graph produces ".pt" extension

make-stoptags.py is checking for `graphbase + '.ht'`

``` python
infiles = [graphbase + '.ht', graphbase + '.tagset']
    if args.stoptags:
        infiles.append(args.stoptags)
    for _ in infiles:
        check_file_status(_)
```

...but loading `graphbase + 'pt'` (and printing ".ht").

``` python
print 'loading htable %s.ht' % graphbase
    htable = khmer.load_hashbits(graphbase + '.pt')
```
