choosenim should update mingw as well
On Windows, different versions of Nim apparently require different versions of Mingw, but choosenim does not have any way to independently update Mingw (or change the Mingw version).

Choosenim should provide a way to update and choose the version of Mingw that is used, not just the version that is installed with choosenim by default, or the system Mingw. 

Perhaps a warning about the system Mingw being too old would also be useful, if a system Mingw is detected.

This is related to #129 and #128 