export variables

I'm sure that you have considered this but I think it would be nice to allow for a syntax that allows for variables.

```hamlet
<div class=#{purescriptVariable}>
  <p>#{anotherPurscriptVariable}</p>
</div>
```

```purescript
[ HH.div [ HP.class_ $ H.ClassName purescriptVariable ]
  [ HH.p_ [ HH.text anotherPurescriptVariable ] ]
]
```