Add libsyn.com to yellowlist

[ghostwords]

Fixes #1180.

More examples:

• https://www.stitcher.com/podcast/rncn/financial-gravity-with-john-pollock/e/53853435
• https://usa.streetsblog.org/2018/03/23/talking-headways-podcast-mobility-is-like-a-heavy-metal-band/
• https://overcast.fm/+HOFj9gH3g

I see session and day-long cookies (#1545):

Error report counts by date and exact blocked subdomain:

+---------+---------------------------+-------+
| ym      | blocked_fqdn              | count |
+---------+---------------------------+-------+
| 2018-03 | assets.libsyn.com         |     3 |
| 2018-03 | fightsgoneby.libsyn.com   |     1 |
| 2018-03 | html5-player.libsyn.com   |     2 |
| 2018-03 | hwcdn.libsyn.com          |     1 |
| 2018-03 | secure-hwcdn.libsyn.com   |     6 |
| 2018-03 | ssl-static.libsyn.com     |     1 |
| 2018-03 | static.libsyn.com         |     4 |
| 2018-03 | traffic.libsyn.com        |    11 |
| 2018-02 | assets.libsyn.com         |     7 |
| 2018-02 | html5-player.libsyn.com   |     6 |
| 2018-02 | hwcdn.libsyn.com          |     5 |
| 2018-02 | my.libsyn.com             |     3 |
| 2018-02 | secure-hwcdn.libsyn.com   |     7 |
| 2018-02 | ssl-static.libsyn.com     |     6 |
| 2018-02 | static.libsyn.com         |     8 |
| 2018-02 | stillbuffering.libsyn.com |     1 |
| 2018-02 | traffic.libsyn.com        |    13 |
| 2018-01 | assets.libsyn.com         |     1 |
| 2018-01 | html5-player.libsyn.com   |     3 |
| 2018-01 | hwcdn.libsyn.com          |     1 |
| 2018-01 | static.libsyn.com         |     1 |
| 2018-01 | traffic.libsyn.com        |     1 |
...