Add support for nullsafe method calls ("?->") to pfff
Add support for nullsafe method calls ("?->") to pfff

Summary:
Hack/HHVM added a new operator "?->" for doing nullsafe method calls.
Nullsafe method calls are the new hotness intended to replace the
"nullsafe()" pattern in www.

Test Plan: make test

Reviewers: pad, julienv, eletuchy, jwatzman

Reviewed By: jwatzman

Differential Revision: https://phabricator.fb.com/D1585372

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