Please include download tool aria2 in LibreELEC

if we are to include all 'very useful' tools, LibreELEC will become so bloated that the vast majority of users will have to reinstall LibreELEC because the system partition has become too small again. Moreover, it's another tool to implement, and test, and support... Right now we are focusing on finalising LibreELEC with Kodi Krypton.