Allow setting custom Imgur API key in .captured.yml

[mathiasbynens]

It would be great if we could just specify our own anonymous Imgur API key in the config file, ~/.captured.yml. Currently everyone with captured installed is using the same API key. An anonymous API key allows up to 50 images per hour, but if I’m not mistaken, currently all captured users are using the same key, meaning if two people each upload 25 images the limit is reached for that hour and all other captured users will have to wait for the hour to pass.

[csexton]

+1 for this, I think this is a great idea, however I did coordinate with the folks at imgur when getting the existing key -- they seem to endorse Captured's usage of it.