[modifier] Boolean modifier

A boolean modifier, that turns into **true/false** javascript _truly_ and _falsy_ values.

**Example:**
Transform
```javascript
{
  trulyNum: 1,
  trulyStr: 'something',
  falsyNum: 0,
  falsyStr: '',
}
```
Into
```javascript
{
  trulyNum: true,
  trulyStr: true,
  falsyNum: false,
  falsyStr: false,
}
```