Add AppVeyor CI config

[k06a]

We need AppVeyor build matrix to support at least 2 axis:

1. Different C++ compilers (gcc, clang, msvc) with different major versions
2. Different C++ standards: C++11, C++14, C++17, C++20 (boolinq-test14 is not compatible with C++11)

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[tylerferrara]

Adding an appveyor.yml configuration file seems strait-forward and well documented for setting up a build pipeline. According to your travis.yml config, this project only targets linux based x86_64 architectures. Do you expect to expand this with AppVeyor? @k06a

As for testing different versions of C++ (Issue: #63), I will communicate with @mostafabarmshory to prevent duplicate work.

[k06a]

Interesting question, I believe it should work for x86, x86_64, ARM and ARM64

[tylerferrara]

Interesting question, I believe it should work for x86, x86_64, ARM and ARM64

We can put this to the test 👍

[tylerferrara]

Update

Adding AppVeyor support to a fork of this project. This includes GitHub integrations to run builds and tests for every PR.

Once the build configuration supports your requirements (compilers & standards), we can create an AppVeyor project for this repo, using the same appveyor.yml.