v1.6.0

Adds:

• cmd.WithUser(syscall.Credential) - Add the ability to change the user a command is run as on *nix systems (#35)
• cmd.WithUser(syscall.Token) - Add the ability to change the user a command is run as on windows systems (#35)

Fixes:

• Ensure we write to stderr when WithStandardStreams option is used. Thanks to @thomasnemer for the fix (#37)