Add cli_bootstrap.php.

Add a CLI specific bootstrap override file. This file is intended to be
used for CLI specific config and bootstrapping. For example, by using
separate log files for web & cli we can avoid annoying file permission
issues. This addresses cakephp/cakephp#2634 and is part of
cakephp/cakephp#3325
Merge pull request #49 from cakephp/cli-bootstrap

Add cli_bootstrap.php.