Feature request: Support executing plugin commands

[shakuzen]

The plugin does what it says with CLI core commands, but it does not appear to work with plugin commands. It would be helpful to combine other plugin commands with this plugin's ability to run the command on many apps.

For instance, if I want to restage all applications in a space without downtime, I might use the bg-restage command from the plugin cf-plugin-bg-restage. However, this does not work with do-all.

$ cf do-all bg-restage {}

API endpoint:   https://***redacted*** (API version: 2.98.0)
User:           ***redacted***
Org:            ***redacted***
Space:          dev
2018/09/13 12:10:16 C0913 12:10:16.039393 19825 plugin.go:154] Error executing cli core command

API endpoint:   https://***redacted*** (API version: 2.98.0)
User:           ***redacted***
Org:            ***redacted***
Space:          dev
panic: Error executing cli core command

goroutine 1 [running]:
panic(0x33cc60, 0xc4201b46c0)
	/usr/local/go/src/runtime/panic.go:500 +0x1a1
github.com/ecsteam/do-all/vendor/github.com/op/go-logging.(*Logger).Panic(0xc420129350, 0xc4201b4610, 0x1, 0x1)
	/tmp/build/80754af9/gopath-tested/src/github.com/ecsteam/do-all/vendor/github.com/op/go-logging/logger.go:188 +0xd0
github.com/ecsteam/do-all/repeater.(*Repeater).runCommands(0xc42012aaa0, 0x560fa0, 0xc42012aab0, 0xc42012b190, 0xb, 0xc42012b588, 0x3, 0xc42000a170, 0x2, 0x2)
	/tmp/build/80754af9/gopath-tested/src/github.com/ecsteam/do-all/repeater/plugin.go:154 +0x522
github.com/ecsteam/do-all/repeater.(*Repeater).Run(0xc42012aaa0, 0x560fa0, 0xc42012aab0, 0xc42000a160, 0x2, 0x2)
	/tmp/build/80754af9/gopath-tested/src/github.com/ecsteam/do-all/repeater/plugin.go:92 +0x4f3
github.com/ecsteam/do-all/vendor/github.com/cloudfoundry/cli/plugin.Start(0x55b4e0, 0xc42012aaa0)
	/tmp/build/80754af9/gopath-tested/src/github.com/ecsteam/do-all/vendor/github.com/cloudfoundry/cli/plugin/plugin_shim.go:30 +0x288
main.main()
	/tmp/build/80754af9/gopath-tested/src/github.com/ecsteam/do-all/main.go:11 +0x56