Add support for timestamps with signed tz offsets

[henridf]

Suricata outputs timestamp like 2015-04-13T11:32:45.143323-0200,
whereas Zeek does 2015-04-13T11:32:45.143323Z. This commit lets
us parse both.

[philrz]

Verified in zq commit af6d47e. Both formats are now accepted.

$ cat types-time.json
{
  "descriptors": {
    "timerecord": [
      {
        "name": "_path",
        "type": "string"
      },
      {
        "name": "ts",
        "type": "time"
      }
    ]
  },
  "rules": [
    {
      "descriptor": "timerecord",
      "name": "_path",
      "value": "timerecord"
    }
  ]
}

$ cat data.ndjson 
{"_path": "timerecord", "ts": "2015-04-13T11:32:45.143323Z"}
{"_path": "timerecord", "ts": "2015-04-13T11:32:45.143323-0200"}

$ zq -t -j types-time.json data.ndjson 
#0:record[_path:string,ts:time]
0:[timerecord;1428924765.143323;]
0:[timerecord;1428931965.143323;]

Thanks @henridf!