[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 related to
bug: https://crosswalk-project.org/jira/browse/XWALK-1565

How to use:
========
$ xwalkctl --continue
Merge pull request #2375 from takethathe/restart_task

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