Add a linter rule for invalid octals
Summary: PHP doesn't handle octals very well. Basically, it seems that any numeric scalar matching /^0\d+$/ will be treated as an octal, whereas this should be /^0[0-7]+$/. As a result, 08 and 09 are both treated as 0 (because they are invalid octals. This diff adds a linter rule to detect this abnormality.
Test Plan: Added unit tests.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D14604