Add ability to pass undefined height in image sizes

[fredrikekelund]

I've been using this module as part of Apostrophe. It does its job very well, but there's one issue (which also relates to the proposed srcset functionality) I'd like to bring up. Namely the fact that I would like to be able to set an undefined height in image sizes. It seems to me like a more common scenario, that you would want to be 100% sure of the width of an image, but let the height be flexible.

I have achieved the same behavior today by setting a very high number for the height parameter. But the behavior of width and height isn't very documented, and passing something like 20000 for the height feels like a hack.