add support for parsing json from stdin

[scottleibrand]

This allows complex-json2csv to be used the same way as json2csv - as the recipient of piped data on stdin. For example:

$ curl -s https://gist.githubusercontent.com/mddub/a34f980c7124ef3f719aa38ff14516ff/raw/6401b4710df4b11e2dd365e9816e0a115dec94ed/loop-devicestatus.json | complex-json2csv
No input file: using stdin
About to convert 2 records from stdin to CSV
device,loop/name,loop/recommendedBolus,loop/enacted/received,loop/enacted/duration,loop/enacted/timestamp,loop/enacted/rate,loop/cob/timestamp,loop/cob/cob,loop/version,loop/timestamp,loop/iob/timestamp,loop/iob/iob,loop/predicted/startDate,loop/predicted/values/0,loop/predicted/values/1,loop/predicted/values/2,loop/predicted/values/3,loop/predicted/values/4,loop/predicted/values/5,loop/predicted/values/6,loop/predicted/values/7,loop/predicted/values/8,loop/predicted/values/9,loop/predicted/values/10,loop/predicted/values/11,loop/predicted/values/12,loop/predicted/values/13,loop/predicted/values/14,loop/predicted/values/15,loop/predicted/values/16,loop/predicted/values/17,loop/predicted/values/18,loop/predicted/values/19,loop/predicted/values/20,loop/predicted/values/21,loop/predicted/values/22,loop/predicted/values/23,loop/predicted/values/24,loop/predicted/values/25,loop/predicted/values/26,loop/predicted/values/27,loop/predicted/values/28,loop/predicted/values/29,loop/predicted/values/30,loop/predicted/values/31,loop/predicted/values/32,loop/predicted/values/33,loop/predicted/values/34,loop/predicted/values/35,loop/predicted/values/36,loop/predicted/values/37,loop/predicted/values/38,loop/predicted/values/39,loop/predicted/values/40,loop/predicted/values/41,loop/predicted/values/42,loop/predicted/values/43,loop/predicted/values/44,loop/predicted/values/45,loop/predicted/values/46,loop/predicted/values/47,loop/predicted/values/48,loop/predicted/values/49,loop/predicted/values/50,loop/predicted/values/51,created_at,pump/pumpID,pump/battery/percent,pump/reservoir,pump/iob/timestamp,pump/iob/iob,pump/clock,uploader/timestamp,uploader/name,uploader/battery
loop://<redacted>,Loop,0,true,0,2016-11-17T05:31:41Z,0,2016-11-17T05:30:00Z,0.2890350848226197,1.1.1,2016-11-17T05:31:41Z,2016-11-17T05:30:00Z,-0.07530898513649975,2016-11-17T05:30:47Z,83,83,83,83,82,81,81,80,79,78,78,77,77,77,76,76,76,76,77,77,78,78,79,80,80,81,82,83,83,84,85,85,86,86,87,87,87,88,88,88,89,89,89,89,89,89,89,89,89,89,89,89,2016-11-17T05:31:41Z,,,,,,,,,
loop://<redacted>,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,2016-11-17T05:31:04Z,<redacted>,100,106.5,2016-11-17T05:31:00Z,0.175,2016-11-17T05:31:00Z,2016-11-17T05:31:04Z,<redacted>'s iPhone,67

[scottleibrand]

I've been using this modification for months to be able to do curl http://some.url | complex-json2csv, but just realized this PR was never merged or published. I think it's ready to do so.