Custom delimiter function

**Proposal**

Support passing `(string, string) => string` function as a delimiter option.

For example, for object:
```javascript
{
first: {
second: "hello"
}
}
```

and `delimiter`:
```javascript
(left, right) => left + right.charAt(0).toUpperCase() + right.slice(1);
```

it will produce camel case names:
```javascript
{
firstSecond: "hello"
}
```