spread

Summary:
Added support for spread arguments f(...[1,2,3]).

Also updated a rule for parsing arrow functions, and added some explanations in
conflicts.txt on why the treatment of arrow functions is limited.

Test Plan:
make tests
./pfff -parse_js ~/www/

Reviewers: pad

Reviewed By: pad

CC: bhosmer, bryano

Differential Revision: https://phabricator.fb.com/D1264313