remove config.mode, allow algo to be a function

Fixes #10, #11 

I'm thinking about something in lines of:

```js
return config.length
    ? (config.length > 0
        ? ret.slice(0, config.length)
        : ret.slice(config.length)
    ) : ret;
```

Then you can just pass a negative `length` to get the last characters:

```js
bust.config({
    algo: function(file) {
        return file.stat.mtime.getTime().toString();
    },
    length: -6
});
```