Accept the "HOST/OWNER/REPO" syntax or passing a full URL for both the
`--repo` flag and the GH_REPO environment variable and allow setting
GH_HOST environment variable to override just the hostname for
operations that assume "github.com" by default.
Examples:
$ gh repo clone example.org/owner/repo
$ GH_HOST=example.org gh repo clone repo
$ GH_HOST=example.org gh api user
$ GH_HOST=example.org gh gist create myfile.txt
$ gh issue list -R example.org/owner/repo
$ gh issue list -R https://example.org/owner/repo.git
$ GH_REPO=example.org/owner/repo gh issue list
GraphQL supports supplying multiple queries in the `query` parameter,
but an additional `operationName` parameter is then required to select
the query to execute.
Previously, it was impossible to pass `operationName` since it would get
serialized under `variables`, but it needs to be a top-level parameter.
With this change, `operationName` is a special GraphQL parameter name
just like `query` already is.
Reading from file via `-F foo=@myfile.txt` syntax would result in
`[]byte` Go type, which by default gets serialized to JSON in base64
format, which we don't want here.
Traverse all parameters and convert any `[]byte` into `string` before
JSON serialization.
