json output would be nice

Following discussions of #17 and #12 I ran into a use case which I think would justify json output.

Turns out ansible supports dynamic inventories determined by invoking scripts, but needs the output of such a script to be in json format:

```
# ansible -i /etc/ansible/hosts.d/cumulus.sh --list-hosts all
[WARNING]: * Failed to parse /etc/ansible/hosts.d/cumulus.sh with script plugin: failed to parse executable inventory script results from /etc/ansible/hosts.d/cumulus.sh: needs to be a json dict
```

If I manually wrap a `nodeattr` invocation in json, I get what I want:

```
# cat /etc/ansible/hosts.d/cumulus.sh
#!/bin/bash
echo '{ "cumulus":["'$(nodeattr -f /etc/genders.d/cumulus.g -c cumulus|sed 's/,/","/g')'"]}'
# ansible -i /etc/ansible/hosts.d/cumulus.sh --list-hosts all
hosts (6):
mgmtsw1
mgmtsw2
mgmtsw3
mgmtsw4
mgmtsw5
mgmtsw6
```

This would be a lot cleaner if I could use `hostlist` to postprocess:

```
#!/bin/bash
nodeattr -f /etc/genders.d/cumulus.g -c cumulus | hostlist -j
```

Such a `-j` option could either add a dummy name (like "hostlist") to the json, e.g.,

```
{ "hostlist": ["host1","host2","host3"] }
```

...or could possibly take a label as an argument: `-j `.

This isn't an urgent priority, but I thought it was a pretty clean example of how other hostname/genders utilities could be retrofitted with a single CLI switch to interact with newer frameworks.

The request is exercised through the hostlist CLI in the shown `hostlist -j` pipeline; start by locating its command-line option handling and output paths. Confirm the intended label behavior, then verify that the result is a JSON object Ansible accepts for dynamic inventory.

Written by the indexing model from the issue text.