[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
There is no condition to return false in this funcion, please re-consider whether it is suitable to define the return value as boolean. Or you need check the `Install()` and `Update()` result to return false on some scenario.