Allow slash separator in Page extract date from URL methods

Summary:
To extract dates from URLs, / and - are valid separators:

• http://.../2016-11-17/...
• http://.../2016/11/17/...

Same for YYYY DD MM format.

Test Plan: Tested with Le Huffington Post change.

Reviewers: dereckson

Differential Revision: https://devcentral.nasqueron.org/D729