Add DKVPX file format

[johnkerl]

Resolves issue #266.

This is like Miller's original DKVP except it handles quoted keys and/or values, like CSV does.

DKVP has a naïve split on = and ,:

$ echo 'a=1,"b,c"=2,d="3,4",e' | mlr -i dkvp -o json cat
[
{
  "a": 1,
  "2": "\"b",
  "c\"": 2,
  "d": "\"3",
  "5": "4\"",
  "6": "e"
}
]

But DKVPX handles these:

$ echo 'a=1,"b,c"=2,d="3,4",e' | mlr -i dkvpx -o json cat
[
{
  "a": 1,
  "b,c": 2,
  "d": "3,4",
  "4": "e"
}
]

Performance

Performance numbers, and why I'm not making DPVKX replace the existing DKVP:

repeat 10 justtime mlr -i dkvp  nothing ~/data/big.dkvp
repeat 10 justtime mlr -i dkvp  cat     ~/data/big.dkvp
repeat 10 justtime mlr -i dkvpx nothing ~/data/big.dkvp
repeat 10 justtime mlr -i dkvpx cat     ~/data/big.dkvp

where big.dkvp is a million-line DKVP file with no quoting on keys or values.

#dkvp-r dkvp-rw dkvpx-r dkvpx-rw
1.102   1.165   1.567   1.650
1.103   1.150   1.579   1.652
1.093   1.151   1.577   1.677
1.096   1.150   1.707   1.660
1.146   1.152   1.602   1.659
1.170   1.139   1.591   1.665
1.106   1.151   1.590   1.665
1.101   1.146   1.583   1.665
1.112   1.145   1.587   1.717
1.113   1.144   1.584   1.677

[aborruso]

Thank you very much!