* Add post images hook
This hook would allow plugins to manipulate images inside posts after images are manipulated by nodebb (for example, delay loading). Without this hook we can manipulate images by searching post data for them and manipulate that data, but any change on that level is either erased by nodebb once image loading starts (if delay is turned on) or it disables native image behaviour by changing image code so native behaviour is not triggered.
Hook at this place works for both delayed loading turned on and off.
Usage for this kind of hook was talked about here: https://community.nodebb.org/topic/10960/opening-uploaded-images-in-modal-windows
* trying to fix Travis error
* Update images.js
Hook moved out of wrapImagesInLinks function to places where images are actually loaded. One of the places is inside unloadImage function but in place where loading is happening because of failed if check (delayImageLoading not se in users configuration).
if the `src` of the `img` does not have an extension, most browsers will trigger a download with whatever filename is set, also without an extension, so when it's opened in any OS, it wont have a default application to open it with.
So, In this case, if the `alt` attribute has an extension (meaning if it looks like filename) - we use that with the html5 [`download` attr](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a#attr-download)
The PR does not affect any src if they have an extension.
Julian Lam
Barış Soner Uşaklı
gasoved
Peter Jaszkowiak
Stjepan Osojnički
Barış Soner Uşaklı
Aziz Khoury