Support for editing objects and lists

[onitake]

When trying to edit a secret that is not a string, I get the following error:

$  vc edit my-secret-object
panic: interface conversion: interface {} is map[string]interface {}, not string

goroutine 1 [running]:
main.WriteSecretToFile(0xc42041e000, 0xc4203fc180)
        /go/src/github.com/adfinis-sygroup/vault-client/src/edit.go:193 +0x43d
main.(*EditCommand).Run(0xc420017d20, 0xc420010350, 0x1, 0x1, 0x0)
        /go/src/github.com/adfinis-sygroup/vault-client/src/edit.go:47 +0x23c
github.com/mitchellh/cli.(*CLI).Run(0xc4200d68c0, 0x0, 0x409125, 0x0)
        /go/src/github.com/mitchellh/cli/cli.go:255 +0x229
main.main()
        /go/src/github.com/adfinis-sygroup/vault-client/src/main.go:32 +0x6f

Please support editing of other secret types, for example by converting them to JSON and back when the editor closes.