WIP:  AVOptions API v0.0 (first step)

AVOptions v0.0  - WIP
=================

New functions and support for AVOptions API in VideoIO
See avoptions.jl for details and examples.  

To do: 
1) Tidying up!
2) Better documentation for the user. 
3) Implement AVDictionary, AVDeviceCapabilityQuery functions.
4) Implementing tests in test/runtests.jl.


It would be nice to have this as a separate pull request, so that it gets applied sooner rather than later.