[PATCH 1 of 3] phabricator: add a contrib script

[Jun Wu]

Excerpts from Siddharth Agarwal's message of 2017-07-02 21:42:04 -0700:
> Does this series mean we can provide a basically equivalent experience 
> to email? (Maybe even better than that because applying changes looks 
> pretty easy?)

Hopefully. I think a major benefit is V2 will be updating existing review
units so reviewers could check the difference between V2 and V1 easily.

One feature I missed is the "threading" view. If we have too many
Differential Revisions listed, it might make sense to have some in-browser
user script to collapse diffs by author.