Gulp.js issue: Cannot set property 'lozad' of undefined

Hi, thanks for the cool library!
Recently faced an unobvious problem

## Current Behavior
When adding a library using gulp.js (concatenate many scripts into one file), I get the error `Cannot set property 'lozad' of undefined`, the error was in this line of code `(global.lozad = factory());` global was undefined
although when loading via cdn - there are no errors

## Possible Solution
I added a simple check `(global = global || self, global.lozad = factory());` and everything worked as expected

Please pay attention

P.S. You've got a cool tool