Add mainClass option for Repackage task #283

This commit adds similar springBoot ext mainClass property
option into Repackage task. If this property is defined within
a task, it works exactly in a same way as if it'd be defined within
a springBoot{} ext properties section.

Option is valid only for that specific task where it is defined,
and will override option defined in springBoot{} ext properties.

It seems that this file was committed into git before eclipse
code formatter was in place, so eclipse added few extra
lines during the save.