Make cache timeout values configurable

[leon-richardt]

Idea spawned from #409 (comment)

[M4tthewDE]

I took a look at how to implement this the best way. Since there are a good amount of different caches (>10), individual config entries for each timeout value would bloat the config quiet a bit.

A possible solution is to use a map:

cache-timeouts:
   youtube: 24h
   twitter: 10m

This is nice in the config.yml in my opinion, but as far as I can see viper does not support maps in command-line flags.
The cache value could as a result only be set via the config.yml.

Not sure which of the two would be the best option, suggestions appreciated!

[pajlada]

We already don't use maps where we could, so I think it makes sense to just add long keys here too
atm we have youtube-api-key, we'd want to add something like youtube-video-cache-duration and youtube-profile-cache-duration, for twitter we'd add twitter-profile-cache-duration and twitter-tweet-cache-duration.

I'm flexible with the naming