So that you can do automatic formatting from an editor. You probably want to
disable the `-write` and `-list` options so that you just get the
re-formatted content, e.g.
cat main.tf | terraform fmt -write=false -list=false -
I've added a non-exported field called `input` so that we can override this
for the tests. If not specified, like in `commands.go`, then it will default
to `os.Stdin` which works on the command line.
e9128769b5