New config lookup

[jdmota]

As discussed in #124, I'm proposing a change in the way we look for .flowconfig, to allow for monorepo folder structure.

Breaking change

• Removed flowDir setting
• Starting on each file, look upwards for a .flowconfig file. That becames the root.

Other changes

• Refactoring and ran prettier
• Removed spec-win script (tested on Windows that spec works just fine)
• Use execa instead of cross-spawn in tests (returns a promise allowing for async tests)

TODO

• Optimize lookup?
• Update README
• Update CHANGELOG

Notes

• spawn for some reason is very slow on NodeJS 10 in Windows. That explains the error in AppVeyor. But also means I can't test locally for now.
• For some reason, it seems that when a test timeouts, the snapshot gets attributed to the wrong test. Async tests that exceeds timeout influence other tests when finally resolved jestjs/jest#5831