Added per environment rsync exclude support

I've added the support for multiple rsync_exclude.txt files, configurable on a per-environment logic, just like config.yml: rsync_exclude.txt is an exclude definition common to all deploy targets and is always included, if it exists and for each deploy target you can define a file named rsync_exclude_{target_name}.txt that will be included only when deploying to that target.
For example, suppose you have 2 targets: _staging_ and _prod_. You can create 2 files, named _rsync_exclude_staging.txt_ and _rsync_exclude_prod.txt_, each excluding the other configuration files and front controller or whatever you want to exclude.