added ability to set the number of logfile lines displayed on errors (number or 'ALL')

According to:

https://github.com/rock-core/autobuild/issues/6
Given the code structure you chose, I would simply initialize @displayed_error_line_count in Subcommand#initialize to the global default instead of having to do it explicitely. Autobuild.displayed_error_line_count is obviously a global autobuild default, so should be used by default by the autobuild code.