Add context to downloading jokes

[helgeolav]

I would like to make Fetch context aware. The simplest way to do so is to create a FetchWithContext that accepts a context as the first parameter. This will make sure that we don't break any existing code. Doing so will result in Fetch calling FetchWithContext with context.Background().

Is this acceptable?

[Icelain]

Thanks for the suggestion. It is a good idea to add a fetch method with context. PRs are always welcome nonetheless I'll add it by today most likely.

[Icelain]

Done with #26