poster: easy retrieval of poster image data

Hi @misterhat,

I made a quick pull request to make retrieval of movie poster easier. But the best part with it is that we can silently change the backend to `img.omdbapi.com` whenever that launches, and thus save the extra request that we currently have to deal with.

Hoping for a quick merge, throw a comment if there is something you don't like and I'll fix it asap!

Cheers :beers: