display issue when percentage drops

When a percentage drops from 100 to 80 or from 30 to 0 (more precisely, when an amount of digits in decimal representation becomes less than it was), old "percent" character remains.

It results in something like this:
```
[          ] 0 % % 
```

Test:
```javascript
var multimeter = require('multimeter');
var multi = multimeter(process);

multi.drop(function (bar) {
    bar.percent(100);

    setTimeout(function () {
        bar.percent(0);
    }, 100);
});
```