Add support to tweet memes

To create twitter keys:

- Go here: https://dev.twitter.com/apps/new
- Login
- Enter application name, description and website.
- Open settings, change from read only to read and write.
- Click "create access token".
- Copy consumer_key, consumer_secret, access_token, and access_token_secret to /etc/iomeme/iomeme.yaml
- Set twitter_enabled = 1 in /etc/iomeme/iomeme.yaml