Added helper functions to render XML elements

[k0ral]

Text.XML.Stream.Parse provides helpers like tag, content and requireAttr, to avoid dealing with bare Events.
On the other side, Text.XML.Stream.Render provides no such tools. This PR tries to fill the gap (also, stylish-haskell went by :) ).

I'm not expecting it to be merged as is, this is somewhat a draft version, please let me know of the adjustments you would require before merging. Some points worth discussing:

• the naming convention: tag, optionalAttr and content will conflict with their parsing homonyms, do we really want that ?
• can we do better/simpler than the Attributes wrapper ?
• does it make sense to provide optionalAttr ?
• can we improve on the haddock ?

[snoyberg]

I very rarely have a need anymore to generate XML in a streaming manner, so I don't have many thoughts on this. The PR looks good enough to me as-is. I don't have any feedback for you right now; would you make whatever changes you'd like and ping me when it's ready, and I'll merge it in?

[k0ral]

I've spent some time testing the new functions, they are working fine so I won't push further changes, you can merge it.
Also, could you please make a new release out of it ? I'm depending on the new functions on some projects.
Thank you.

[snoyberg]

Merged and released to Hackage and 1.2.5, thanks!