Implement a tool to extract segments (morphemes) from .dix files

In addition to #78, it would be great to have a tool, let's call it `lt-segment` that would calculate a segment vocabulary from a `.dix file`. E.g.

```
...

s

ouseouse

iceouse

yy

iery

iesty

cat
bat
happ
eas
m
l
```

Would produce something like
```
cat bat happ eas m l @s @ouse @ice @y @ier @iest
```
It could also be good to have the frequency.