Allow to read Phabricator token

Summary:
Can be used for API queries:

curl https://devcentral.nasqueron.org/api/... -d api.token=$(phabricator/read-token.py) ...

Test Plan: Tested against devcentral with diffusion.repository.search query.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2470