Add support for parsing EXT-X-PROGRAM-DATE-TIME

[ryanische]

Makes the raw value of the EXT-X-PROGRAM-DATE-TIME available on a playlist item. I used the same property name as what was added in #2.

[RWOverdijk]

👍 Can we have this please?

[RWOverdijk]

Your solution breaks for me though:

/Users/rwoverdijk/projects/nodejs/node-m3u8/parser.js:95
  this.currentItem.set('date', data);
                  ^

TypeError: Cannot read property 'set' of undefined