bug: json value in .grant.yaml doesn't work

Hi,

I tried to run the binary using a simple config file as specified in the documentation:
```
#.grant.yaml
config: ".grant.yaml"
format: json # table, json
show-packages: false # show the packages which contain the licenses --show-packages
non-spdx: false # list only licenses that could not be matched to an SPDX identifier --non-spdx
osi-approved: false # highlight licenses that are not OSI approved --osi-approved
rules:
- pattern: "*gpl*"
name: "deny-gpl"
mode: "deny"
reason: "GPL licenses are not allowed per xxx-xx company policy"
exclusions:
- "alpine-base-layout" # We don't link against this package so we don't care about its license
```

run the below line:
`grant check redis:latest`

I got table output instead of json output.

I think it will be better that the configuration will be "stronger" than command line (in case it is exists)
moreover, I used -c flag like this:
`grant check redis:latest -c ./.grant.yaml`
`grant check redis:latest -c .grant.yaml`

still printed it as a table.

Thanks a lot for your time!