Add support for begin_export html

[pierre-lecocq]

As described by Bastien Guerry in https://github.com/bdewey/org-ruby/issues/73 , we need to support #+begin_export html.

Here is the code to do so!

Thanks

[wallyqs]

Thanks for the contribution! Apologies on the delay! I seem to have missed the notification from this repo, but now picking up the change into my fork at https://github.com/wallyqs/org-ruby I confirmed this is green locally for me by updating the spec so will take a look at cutting a new release with the change soon https://github.com/wallyqs/org-ruby/commits/begin-export-support-fixes

Thanks!