[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
> 1. DLOG/DVLOG instead of g_print?
In xwalkctl_main.cc, always use g_print to print information, and it will treat the i18n cases automatically.
> 2. Should not return if continue_tasks is true?
Yes, xwalkctl try to support this usage mode: ```xwalkctl -c -i /path/to/package.wgt```, so do not return immediately here.