Pass options to `Builder::XmlMarkup`

This PR adds option `:builder` which is directly passed to `Builder::XmlMarkup`
So it's possible to pass options like `:indent` and others. For example
```ruby
Gyoku::xml({:some => { :new => "user" }}, {:builder => { :indent => 2} })
```
will produce
```
<some>
  <new>user</new>
</some>
```

To make indentation work correctly with `Builder` I had to make changes in that gem, so this won't work unless jimweirich/builder#44 is merged