Update the `plan` command.

[angrycub]

This is a pretty big PR, but this fixes the levant plan command to output an entire plan in "pretty" mode. Also added some CLI flags to allow the user to specify the output format and where they would like the output to go. I added a CLI mode in case we wanted to bring colors to the party.

I think the JSON -> Lines conversion could probably be made more elegant :ring:, but for now it seems to do the right thing. Would like to add some testing for all this too, but I wanted you to start milling over what I'd done thus far.

[hashicorp-cla]

All committers have signed the CLA.