[piksel] add subtitle capability (e.g. https://player.piksel.com/v/ums2867l)

[chuckcho]

Please follow the guide below

• You will be asked some questions, please read them carefully and answer honestly
• Put an x into all the boxes [ ] relevant to your pull request (like that [x])
• Use Preview tab to see how your pull request will actually look like

---

Before submitting a pull request make sure you have:

• At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
• Searched the bugtracker for similar pull requests
• Checked the code with flake8

In order to be accepted and merged into youtube-dl each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

---

Description of your pull request and other information

Added subtitle feature for piksel, e.g. now supporting youtube-dl --write-sub --sub-lang en --skip-download 'https://player.piksel.com/v/ums2867l'

[remitamine]

the test doesn't pass:

python test_download.py TestDownload.test_Piksel
[Piksel] ums2867l: Downloading webpage
[Piksel] ums2867l: Downloading JSON metadata
[Piksel] ums2867l: Downloading m3u8 information
[info] Writing video description metadata as JSON to: test_Piksel_ums2867l.info.json
[debug] Invoking downloader on 'http://usccupd.edgesuite.net/mm/flvmedia/3811/g/l/t/gltap28a_f350s7l0_h264_1048K.mp4?cid=3811&aid=3238987&afid=6704067&assetid=3238987'
[download] Destination: test_Piksel_ums2867l.mp4
[download] 100% of 10.00KiB in 00:01

'info_dict': {
    'id': 'ums2867l',
    'title': 'GX-005 with Caption',

    'upload_date': '20161210',
    'timestamp': 1481335659,
},
F
======================================================================
FAIL: test_Piksel (__main__.TestDownload):
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_download.py", line 210, in test_template
    expect_info_dict(self, tc_res_dict, tc.get('info_dict', {}))
  File "/home/amine/youtube-dl/test/helper.py", line 226, in expect_info_dict
    ', '.join(sorted(missing_keys))))
AssertionError: {'upload_date', 'timestamp'} is not false : Missing keys in test definition: timestamp, upload_date

----------------------------------------------------------------------
Ran 1 test in 13.306s

FAILED (failures=1)

[chuckcho]

fixed

the test doesn't pass:

python test_download.py TestDownload.test_Piksel
[Piksel] ums2867l: Downloading webpage
[Piksel] ums2867l: Downloading JSON metadata
[Piksel] ums2867l: Downloading m3u8 information
[info] Writing video description metadata as JSON to: test_Piksel_ums2867l.info.json
[debug] Invoking downloader on 'http://usccupd.edgesuite.net/mm/flvmedia/3811/g/l/t/gltap28a_f350s7l0_h264_1048K.mp4?cid=3811&aid=3238987&afid=6704067&assetid=3238987'
[download] Destination: test_Piksel_ums2867l.mp4
[download] 100% of 10.00KiB in 00:01

'info_dict': {
    'id': 'ums2867l',
    'title': 'GX-005 with Caption',

    'upload_date': '20161210',
    'timestamp': 1481335659,
},
F
======================================================================
FAIL: test_Piksel (__main__.TestDownload):
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_download.py", line 210, in test_template
    expect_info_dict(self, tc_res_dict, tc.get('info_dict', {}))
  File "/home/amine/youtube-dl/test/helper.py", line 226, in expect_info_dict
    ', '.join(sorted(missing_keys))))
AssertionError: {'upload_date', 'timestamp'} is not false : Missing keys in test definition: timestamp, upload_date

----------------------------------------------------------------------
Ran 1 test in 13.306s

FAILED (failures=1)

[remitamine]

Thanks.