improve cache being used for dns lookups

[vishen]

Currently the description of the --cache flag is very unhelpful. This should be better documented in the cmd flags and possibly the readme. There should also be a command to manually clean out the cache entries in-case someone wants to clear out the cache.

I ran into an issue I had a non-local ip address cached for one of my chromecast dns entries. It would be good to remove or add a short TTL on each cache entry so a bad entry will eventually be expired.