Also accept a function as a prop transformer

This is just a suggestion, as it can lead to simpler code.
For example, with redux you can map the state to props via both simple objects or (for more complex use cases) functions.

Here's an example:
```
mapProps(props => {
const newProps = {}

if (someExternalService()) {
newProps.foo = 'bar'
}

return newProps
})(Component)
```

While I know you can achieve the same result using only objects, it's a nice to have.

BTW, nice work on this small handy HOC 👍