Add color output support for supported platforms on Windows

Modifies the `disableANSI` code to not outright reject Windows
but check the `TERM` environment variable in case it supports
ANSI colors. This applies to CygWin, PowerShell and integrated
terminals such as the ones in IntelliJ IDEs.

Tested manually in PowerShell, ConEmu and `cmd.exe`.

See: <https://github.com/facebook/libphutil/pull/30>

Reviewed by: epriestley