Use yml files to list commands

[marcelovani]

We have hard coded the commands to be executed, see
https://github.com/dennisinteractive/drupal_console_commands/blob/master/src/Command/Site/BuildCommand.php#L63
and
https://github.com/dennisinteractive/drupal_console_commands/blob/master/src/Command/Site/UpdateCommand.php#L56

It would be nice to have yml files to list these commands.
We also need a plugin that detects the yml files on local folders, i.e. docroot or the global (~/.console/commands/) for example. These files would take precedence over the default yml with commands.