Use chcp only if sed is in PATH

[janlazo]

Fix #884

sed is now required to parse chcp output. Vim can be used instead via separate child system call but doing it per s:system makes Windows slower. chcp parsing is fragile because of the system locale. There's no convenient way to parse out the codepage value without regex just by relying on cmd.exe builtins and default binaries in PATH.
If this still doesn't work, users will have to get by with ascii output only (no readable unicode symbols from console) because this is a OS problem. This applies to fzf as well because the chcp code orignated from fzf Vim plugin was meant for UTF-8 characters.

Powershell builtins could be used as an alternative to sed but using powershell on non-powershell &shell slows down s:system.

[janlazo]

This assumes that the first numeric token is the codepage.