Add network request poll frequencies to config file

[hawkins]

I.e.,

• online-poll-frequency
  • How often we refresh the online/offline list
• status-poll-frequency
  • How often we refresh each streamer card's status and game
• viewer-poll-frequency
  • How often we check for how many viewers, views, and followers a channel has

And just write them each as keys in the ~/.streamflow.json file with units as milliseconds, like so:

{
  favorites: [...],
  "online-poll-frequency": 60000,
  "status-poll-frequency": 60000,
  "viewer-poll-frequency": 5000
}

Maybe want to shorten the names, however