allow for non-string outputs

[jonasi]

not sure if this is new in 0.7, but non-string outputs are allowed. Without this fix, I get

panic: interface conversion: interface is []interface {}, not string

goroutine 1 [running]:
panic(0x1020e0, 0xc420498e00)
        /Users/isao/.gimme/versions/go1.7.darwin.amd64/src/runtime/panic.go:500 +0x1a1
main.(*state).outputs(0xc4200722c0, 0xc420357bc0, 0xc4202cdd00, 0xc4202cdd50)
        /Users/isao/dev/src/github.com/adammck/terraform-inventory/parser.go:41 +0x3b0
main.gatherResources(0xc4200722c0, 0xc4a44)
        /Users/isao/dev/src/github.com/adammck/terraform-inventory/cli.go:23 +0x3a3
main.cmdList(0x19f4c0, 0xc420098008, 0x19f4c0, 0xc420098010, 0xc4200722c0, 0x0)
        /Users/isao/dev/src/github.com/adammck/terraform-inventory/cli.go:33 +0x2b
main.main()
        /Users/isao/dev/src/github.com/adammck/terraform-inventory/main.go:63 +0x4e4

[adammck]

Thank you, @jonasi! I've been meaning to fix this for a while. Would you mind adding a few examples to this test, so we can be sure not to re-break this in future?

[jonasi]

@adammck updated with a list and map example

[adammck]

Wonderful, thank you! I don't know that the output format will actually be useful to Ansible (do you?), but this solves the crash nicely. Thanks for the patch!