Add Windows to the CI build

[iamdefinitelyahuman]

The Issue

We cannot run the full test suite on Windows because of issues with pyethash (Consensys/ethjsonrpc#5), which is a requirement for pyevm (ethereum/py-evm#395).

The Solution

Create a Windows build that runs a limited set of tests to verify the functionality of the CLI. This is the only area of the code base that touches the file system - the most likely area for cross-platform issues.

Originally discussed at #1781 (comment)

[fubuloubu]

As a part of this, the CLI tests would need to be their own separate part of the CI process to allow this.