Add pre-commit hook - #4644

This makes it easy to add Phan to a project using the pre-commit framework.

If pre-commit, PHP and Composer are installed, only a file .pre-commit-config.yaml is needed in the project:

repos:
- repo: git://github.com/phan/phan
  rev: e56968f2b00d19c3c8d3ee113b9d7255eca7cccd
  hooks:
  - id: phan