Simple method to update cats data

[Cowa]

To minimize Imgur API quota, we currently make only one Imgur API request on the first use of the resources. Then every resources are stored in memory.

If new cats appear in the Imgur main album, they will not be available until the next reboot :(

The goal here is not to have an automatic update when a new cat is inserted in the Imgur main album.
But only a simple method that will re-fetch cats from Imgur.

Later, it'll be used by CatApi.

Extra-info :
cats will no longer be a lazy val, but only a var (see it)

[Cowa]

It's gonna change with #7