Added boolean handling for the checked attribute

this allows to conditionally generate the checked attribute on radio buttons or checkboxes.

example:
```jade
input(type="radio", name="test", value="1", checked=checkedOne)
input(type="radio", name="test", value="2", checked=checkedTwo)
```

if $checkedOne is set to a truthy value, the checked attribute will be added to the generated php otherwise it is omitted completely.