update url to generate paths with anchors

This updates the `Url` class to make it generat paths with anchors.
Pass in a `#` key to the hashed args and its value will be appended
onto the path as an anchor.

For example:

```ruby
url = Url.new('an-url', '/path/to/:value')
url.path_for({
  'value' => 'thing',
  '#' => 'this'
}) # => '/path/to/thing#this'
```

@jcredding ready for review.