Change of 'word' breaks 'before' param

Hi,
I tried to switch to 0.4.0 and the change of fetching method for `word` (https://github.com/f/omelette/commit/b74b23209f5b8254f0d28b120fdfbf034086c30c#diff-564d24cceffda49c18bf427e92b8baa3R23) makes the parameter `before` undefined when an incomplete word is tabbed.

Ex:
```
lorem -> a1 b1 👍
lorem a -> a1 👍
lorem a1 -> a2 b2 c2 👍
lorem a1 a -> nothing 👎
```

My script that goes like this
```js
const autocomplete = {
'a1': {
'a2': {
'a3': {
'a4': {},
'b4': {}
},
'b3': {}
},
'b2': {},
'c2': {}
},
'b1': {
'a2': {},
'b2': {
'a3': {
'a4': {},
'b4': {}
},
'b3': {}
},
'c2': {}
}
};

const completion = omelette('lorem');

completion.on('$1', ({ reply }) => {
reply(Object.keys(autocomplete));
});

completion.on('$2', ({ reply, before }) => {
try {
reply(Object.keys(autocomplete[before]));
} catch (e) {}
});

completion.on('$3', ({ reply, before, line }) => {
const [, lvl1] = line.split(' ');
try {
reply(Object.keys(autocomplete[lvl1][before]));
} catch (e) {}
});

completion.on('$4', ({ reply, before, line }) => {
const [, lvl1, lvl2] = line.split(' ');
try {
reply(Object.keys(autocomplete[lvl1][lvl2][before]));
} catch (e) {}
});

completion.init();
```