Make things configurable, README update

I made a bunch of things configurable: 

```ruby
set :foreman_roles, %w(app) # roles to run capistrano-foreman on
set :foreman_application, -> { fetch(:application) } # name to use when exporting/starting/stopping Foreman services
set :foreman_user, -> nil # username to use when generating services; this defaults to the user that Cap's using for SSH.
set :foreman_log_path, -> { shared_path.join('log') } # log path, defaults to your deployment's /path/to/cap/shared/log
```

In the task  `host.user` is used unless `:foreman_user` is set. 

I added these to the README. While in the README I added syntax highlighting and clarified that this specifically is written for Upstart - not any of the other export types that Foreman supports.