Check shell in Windows before assuming cmd.exe

In Windows, some users set the Cygwin shell as their Vim shell, consider
this before rewriting `cd` calls. Checking if `shellcmdflag` contains a
backslash should catch all true windows shells.

You can also check if &shell contains `sh`, but this seems more robust.
Merge pull request #17 from Chronial/fix-win-shell

Check shell in Windows before assuming cmd.exe