Cache requests from older periods

[HugoJF]

After the update I released attempting to circumvent Twitch's weird API pagination that lead to lots of clips missing from bigger channels, fetching the every single clip from a big channel started taking a long time.

The idea is to locally cache requests for clips from periods older than the current day, that way the tool can be restarted without having to paginate the entire API again.

[HugoJF]

This was partially implemented in 51f860e.

It currently caches every call to the fetchClips() function in cache/{streamerId}-clips, where each call (currently a period of 7 days) is stored in it's own file.

A few improvements for the future:

• Transparent cache implementation, it's currently a bit too verbose;
• Don't cache the most current period;
• Use actual stream name in the cache directory;
• Handle exceptions.