[Runtime] Add "-c" option for xwalkctl for restarting aborted install/update tasks

Crosswalk runtime should have a mechanism to restart the aborted install or
update tasks which are stopped by accident like power off. This patch will
enable this feature, user can restart these tasks by command 'xwalkctl
-c|--continue'. This patch related to bug:
https://crosswalk-project.org/jira/browse/XWALK-1565
From the description, I think `ContinueUnfinishedTasks` should be more accurate name.