Skip AppVeyor in some automated commits

[ocefpaf]

@pelson I believe we don't need to run AppVeyor for some automated commits like this one:

https://ci.appveyor.com/project/conda-forge/staged-recipes/build/1.0.124

Can we add a [skip appveyor] string to that commit message?

[pelson]

Yeah. Good idea. I think there are other things we can do too.

https://github.com/conda-forge/staged-recipes/blob/master/.CI/create_feedstocks.py#L126

[jakirkham]

Also, added a request for Circle CI to support more specific commit skip syntax so we can try that too.
( https://discuss.circleci.com/t/skip-ci-commit-message/2770 )
( https://discuss.circleci.com/t/adding-circle-skip-to-commit-message-to-skip/2778 )

[jakirkham]

This issue appears to be referenced before, but is the same idea for Travis CI ( travis-ci/travis-ci#5032 ). Though, admittedly, we don't need this here.