[feature] add thunking API

In `yo-yo` and `choo` it's a best practice to have element functions in an
application return the same element given the same input. This mechanism is
commonly referred to as `thunking`.

How would we feel about adding a light wrapper that exposes a `thunk` function
so that elements can be thunked with little effort. I was thinking of an API
along these lines:

``` js
// elements/my-button.js
const thunk = require('bel/thunk')
const html = require('bel')

module.exports = thunk(createMyButton)

// create a cool button that has a different color
function createMyButton (color) {
return html`

Click me! :D

`
}
```

In a `choo` app this could then be consumed as:

``` js
const html = require('choo/html')
const choo = require('choo')

const myButton = require('./elements/my-button')

const app = choo()
app.router([ '/', myView ])

app.model({
state: { color: 'blue' }
})

const tree = app.start()
document.body.appendChild(tree)

function myView (state, prev, send) {
return html`

${myButton(state.color)}

`
}
```

What do you think? Would this be a useful addition to `bel`, or should we
continue to point people to use an external thing / write it themselves? I was
thinking of including this into `choo`, but given that we strongly encourage
people to use `bel` for standalone components (yay, reusability) I figured it
might make more sense to include it here first, and then have it propagate
upwards through `yo-yo` to `choo`. Thanks!
### See Also
- https://github.com/Raynos/vdom-thunk