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