Add option to load photoswipe.dev library

Thanks @dinazaur I left some comments. :)

And please add tests to show it works as expected now and in the future.

There is no "dev" version of photoswipe library when installed via bower/npm assets. So we should change the target branch for 3.x, since 5.x will not include this functionality.

@Grevil: I think it's not worth to implement new features in 4.x - so let's close this then... if someone needs it, this can be reopened with a MR.

@dinazaur or would you like to have this merged into 4.x (former 3.x)?

Hi @Anybody, as this is a minor addition, and probably no one will use we can always re-open the issue again.