DO NOT MERGE UNTIL SECURITY PROBLEMS ARE RESOLVED!

Fixes #734

This PR is an attempt to provide Windows builds as proposed at #734

Prerequisites:

• chocolatey
• llvm (Travis uses v9.0.0)
• make (install using choco install make)

Build:

refreshenv
cmake . -B build -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -G "Unix Makefiles"
cmake --build build --target jsonnetfmt
cmake --build build --target jsonnet
cmake --build build --target run_tests

.travis.yml is modified so that archives with executables can be found at directory upload

Problems:

• -pedantic leads to compiler errors since MSVC libraries use language extensions
• -std=c++11 leads to compiler errors
• unable to run setup.py
• unable to run regression tests (CMake target run_tests, filetest_suite/run_tests.sh)
• Secrets ARE NOT obfuscated in Travis Windows builds (means this PR is unable to merge)

Any support is appreciated.