Prepend selector classes to attribute class

With this change, setting an element class with both a CSS selector and an attribute combines the two, instead of clobbering the attribute class. For example:
```coffee
div '.foo', 'class': 'bar'
```
yields
```html
<div class="foo bar"></div>
```
instead of 
```html
<div class="foo"></div>
```

This is useful when creating tag-like helpers that add default classes:
```coffee
{button, normalizeArgs} = require 'teacup'

submit = ->
  {attrs, contents} = normalizeArgs arguments
  attrs.type = 'submit'
  button '.btn', attrs, contents

submit '.place-order', 'Order'
```
yields
```html
<button class="btn place-order" type="submit">Order</button>
```