Add :scm_path configuration option

`:scm_path` configuration option allow to define in which directory to look for custom scm.

Example: to use the custom scm file *my_scm.rb* located in your application's *lib/capistrano* folder, set in your config.rb the 2 directives:

```
set :scm, :my_scm
set :scm_path, 'lib/capistrano'
```