Add a -j / --join-output option.

I'm processing logfiles for [Docker](http://docker.io/); their JSON format looks like this:

```js
{"log":"Sat Nov 16 18:44:01 UTC 2013\n","stream":"stdout","time":"2013-11-16T18:44:01.218661284Z"}
{"log":"Sat Nov 16 18:44:02 UTC 2013\n","stream":"stdout","time":"2013-11-16T18:44:02.222586697Z"}
```

I use `jq -r .log`, but the output includes blank lines. This trivial pull request adds a `-j / --join-output` option to remove the extra `\n` output by `jq` after processing each JS element.

I didn't find tests for output options; so let me know if I missed them and should write a test case for this (I might need some guidance then :-))

Thank you for making `jq`!