Windows Support for asciinema

[cretz]

• Initial request closed at https://github.com/asciinema/asciinema/issues/47
• Could probably use Win32 functions for this. Maybe FreeConsole first, then AttachConsole with ATTACH_PARENT_PROCESS, then maybe able to use regular Golang stdin/out/err (otherwise use GetStdHandle or something maybe)
• Might need a child process instead of doing this right in the asciinema process
• Actually, reviewing how it starts a new shell, we could easily just start cmd.exe and steal that info

[cretz]

Look what's coming: https://blogs.msdn.microsoft.com/commandline/2018/08/02/windows-command-line-introducing-the-windows-pseudo-console-conpty/