Introduce :convert_dashes_to_underscores option

Currently nori will automatically convert all dashes in tag names to underscores. 

```ruby
Nori.new.parse('<hello-world>Hi</hello-world>') # => " {"hello_world"=>"Hi"}
```

We want to use nori to test XML responses of our API. Some of our tag names contain dashes and some underscores. We would like to make the conversion behavior configurable.

This pull request introduces the `:convert_dashes_to_underscores`. It's enabled by default to not be a breaking change.