[opensuse-buildservice] [PATCH] get build log starting from the offset

Hi, When I fix some building errors on our build service, it's very painfull to wait for loading all the build log especially Firefox's 38M bytes log, while I only want to see the last few lines where the error meets. Luckily, web API provides a parameter to set the offset which is not included in the CLI, so I just add an option for the CLI to set the it. Moreover, I hope osc can provide an API & option for only access the last lines of build log. From my experience, it makes sense for 80% time. BR, Mingxi