Add an "--ignore-duplicates" flag to import_project_symbols.php

Summary: People are hella lazy and don't want to do this themselves.

Test Plan: Generated a symbol file with duplicates and piped it in, got an import under --ignore-duplicates.

Reviewers: kdeggelman, btrahan, vrana, jungejason

Reviewed By: jungejason

CC: aran

Differential Revision: https://secure.phabricator.com/D2145