Add Drush command to reload translations from local files

At Sutharsan's recommendation in #1430004 I'm creating this new ticket.

Translated strings for addons are currently only loaded when the addon is installed. If custom modules are deployed then it is very difficult to update strings for those modules. Therefore, I need a feature which forces translated strings to be reloaded.

I've implemented this feature as a Drush command "i10n-reload" in a custom 6.x module. I have no real preference if this feature remains a separate command or is somehow included in the current "i10n-update" Drush command. My implementation is attached.