Support for "append" mode for Azure Blobs

[geovalexis]

Hi all!

I use smart-open for one of my projects and I've recently run into the need for "append" mode for Azure blobs. This is something Azure's SDK supports natively but it looks like it hasn't been implemented in smart-open yet.

I was thinking on adding support for this feature myself but I was wondering if there is any additional concern/inconvenience I might be missing.

P.S.: Thanks for such a simple yet useful tool!

Cheers.