R1:8883c9494fef

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…