Allow specifying custom celerity resource map

Summary:
We have custom static resources.
We currently include them in Phabricator's celerity resource map which is causing some pain - we need to regenerate the file without our custom resources before pushing upstream, we need to discard our changes before pulling from upstream and we need to rebuild with our changes to run Phabricator.

This diff allows writing and reading the map in other location.
The plan is this - I will run celerity_mapper.php twice - once to build Phabricator-only resources (to push to upstream) and once to build Phabricator + ours resoruces to put in our directory.

Better solution would be to create a map just with our resources and read and combine it with Phabricator resources.
But it is complicated because we have dependencies on Phabricator resources.

Test Plan:
celerity_mapper.php webroot
celerity_mapper.php webroot ../facebook/src/__celerity_resource_map__.php
Delete Phabricator's celerity map, set 'celerity.resource-path' and successfully load Phabricator.

Reviewers: epriestley, edward

Reviewed By: epriestley

CC: aran, Koolvin

Maniphest Tasks: T721

Differential Revision: https://secure.phabricator.com/D2630