proposal: on 64-bit Windows, download 64-bit MinGW & Nim
As far as I understand, choosenim currently [just downloads a 32-bit version of MinGW on Windows](https://github.com/dom96/choosenim/blob/121133ba06b0954e058d38743a18fe1ad1f817f2/src/choosenim.nim#L36) (if no gcc is found in PATH), and then, as a consequence, a 32-bit version of Nim. Is there any interest in allowing choosenim to download a 64-bit version of MinGW (and Nim)?

If yes, should there be a flag for choosing the variant, or should the 64-bit toolchain be the default one on 64-bit Windows ([autodetected](https://blogs.msdn.microsoft.com/david.wang/2006/03/27/howto-detect-process-bitness/))?