Add support for customToken

[tuananhnguyen-ct]

Fixes https://github.com/humio/cli/issues/157

[tuananhnguyen-ct]

@SaaldjorMike @jespermbyskov anyone can have a quick look please?

[SaaldjorMike]

The most recent changes refactors how we interact with GraphQL for the humioctl tool. Going forward, you have to

1. update https://github.com/humio/cli/tree/master/internal/api/humiographql with any new interaction with the LogScale GraphQL API
2. generate Go-code based on the updated GraphQL queries/mutations: go generate ./...
3. update humioctl within https://github.com/humio/cli/tree/master/cmd/humioctl so it uses the newly generated code

Right now the code path is as follows: https://github.com/humio/cli/tree/master/cmd/humioctl calls https://github.com/humio/cli/tree/master/internal/api which in turn calls the autogenerated code in https://github.com/humio/cli/blob/master/internal/api/humiographql/humiographql.go.