Add Avro file output and test

[aosagie]

Hi,

After my team and I wondering (for a while) why we couldn't use avro-tools to open the output of this project, we stumbled upon this issue: #15

This PR addresses it by creating a flag to output in the Avro file format.

I'm new to the internals of this project so I may not have done it in the most optimal way.

Thanks for a decade of building and supporting such a great project.

[cowtowncoder]

@aosagie Excellent! Thank you for providing this patch -- it does make sense.
I think it may be possible to slightly optimize this, but I think simplicity of the approach is good as the first step.
So I would be happy to merge this patch, to be included in 2.9.0 (since it requires API add which can't be done in a patch version).

One small process thing we need is to get a Contributor License Agreement (CLA). It only needs to be done, and is good for all further contributions for Jackson projects.
Document itself is at:

https://github.com/FasterXML/jackson/blob/master/contributor-agreement.pdf

(this is individual one; there's corporate one too if need be, but most contributions are under individual one)

and the usual method is to print, fill & sign, scan and email to info at fasterxml dot com.
So if you could do this I would be happy to get PR merged ASAP.

Thank you once again.

[aosagie]

Sounds good. I've emailed the signed document.

[cowtowncoder]

CLA received; will merge.