add ability to export log as json

[masukomi]

hey log-json <duration>

Basically the same thing as hey log <duration> but it would output a JSON array with containing a denormalized object / Hash for each event that looked like this.

events that had not ended /would/ be included, but would have null in the ended_at field.

{
  "type": "event",
  "id": 12345,
  "created_at": "<ISO-8601 string>",
  "updated_at": "<ISO-8601 string>",
  "started_at": "<ISO-8601 string>",
  "ended_at": "<ISO-8601 string>",
  "total_seconds": 12345,
  "projects": [ "foo", "bar"],
  "tags": ["baz", "beedle"]
}