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