Self chat from messages in file

[spencerp]

Patch description
I found myself wanting to generate responses for long list of initial messages. To do this, I extended the self chat to allow for starting the chats with messages from a supplied file.

Testing steps

λ printf "hello\nhowdy" > ~/tmp/openers.txt
λ parlai self_chat -mf zoo:blender/blender_90M/model --seed-messages-from-file ~/tmp/openers.txt --selfchat-max-turns 2 --num-self-chats 2
15:53:34 | creating task(s): self_chat
[context]: Hi!
[TransformerGenerator_1]: howdy
   [TransformerGenerator_2]: hi , how are you doing today ? i just got back from a long day at the office .
-- end of episode --
15:53:35 | 50.0% complete (1 / 2), 0:00:00 elapsed, 0:00:01 eta
    exs  gpu_mem
      1   .01193
[context]: Hi!
[TransformerGenerator_1]: hello
   [TransformerGenerator_2]: hello , how are you today ? i just got back from a long day at work , so i ' m nervous .
-- end of episode --

[stephenroller]

Lgtm. Add a test

[stephenroller]

Cheers mate