add support for exporting falsey values

i'd like the ability to export `false` rather than a function which returns false.  for example:

**export-boolean.js**:

```js
var has = feature_detect()

module.exports = has
```

rather than:

**export-function-which-returns-boolean.js**:

```js

var has = feature_detect()

module.exports = function(){
  return has
}

```