I currently have an Anker 5 port USB charger, though this one would work too -
Both are 110-240v. There are similar devices that include a power bank, but those cost more and I have not tried them.
Add an outlet converter and you're all set. Either the $2 single-type, buy one Euro, one UK, etc, or one of the multi-plug ones.
Some of the multi-adapters also have multiple USB ports, but I tend to think these will be fiddly bits of cheap plastic that will die.
For US to EU plugs, for example, this looks interesting (6 pack, $8)
I have this one - it's a three pack, and converts one EU plug to two US plugs