Set screenshot dir

[marcdel]

Implements https://github.com/HashNuke/hound/issues/196

WIP: line 66 of screenshot_test.exs is failing because I haven't found a way to get the ConnectionServer to pick up the updated :screenshot_dir config value since the supervisor has already started it. I've tried changing sessions, setting the env variable, stoping/starting the agent (my limited experience with OTP kept me from successfully going down this path, but I assume there's an easier way anyway).

If you know of a good way to update those config values, or have a better way of testing that, I'm definitely open to suggestions 😀