Add Windows CI (appveyor)

[jameslamb]

Currently, we have no tests on whether the R or Python packages work on Windows. Should add some CI using Appveyor to test this. See for example https://github.com/jameslamb/rsixygen/blob/master/appveyor.yml

It would be valuable to do this with two PRs:

1. (easier) test that the libraries can even be installed ~2. (harder) test the libraries against actual running Elasticsearch~

~For whoever picks up number 2...I don't think it's worthwhile to test against all of the historical versions. It's sufficient to only test on windows against the most recent version of Elasticsearch.~

References

Theoretically it's possible to run Elasticsearch on Windows: https://www.elastic.co/guide/en/elasticsearch/reference/current/windows.html

[jameslamb]

To close this issue, I actually think it would be ok to just install the package, set whatever environment variable tells testthat that it's on CRAN, and then runs the tests. The integration tests would all be skipped (only unit tests would be run) but that would still be a big improvement over the current state.

In other you do not need to set up Elasticsearch on Windows to address this issue