Add a --format=raw option for eval (#3207)

This is a very simple addition but it's proven extremely useful to us since
it means you can now use `opa` easily in bash scripts, e.g.:

```bash
ACCOUNT_ID="$(opa eval -d accounts.rego --format raw "data.accounts.account")"
```

This means `accounts.rego` can be used by policies as well as e.g. deploy
scripts.

The naming and functionality is inspired by the `-r` flag from `jq`.  In fact,
`jq` can be used to replicate this behaviour, but it is a bit nicer to not rely
on that system dependency.

When multiple queries and expressions are given, they are printed in a simple
table format using single newlines and spaces since that is consistent and plays
nice with `bash`.

Signed-off-by: Jasper Van der Jeugt <jasper@fugue.co>