Create lt.yaml

Lithuanian holidays
Update index.yaml

Added lt.yaml
Merge branch 'patch-2' of github.com:Brunas/holidays into Brunas-patch-2
Merge branch 'patch-3' of github.com:Brunas/holidays into Brunas-patch-2
Tidy up LT defs
Merge LT defs from 'Brunas-patch-2' and 'Brunas-patch-3'
Add LT