Refactor inline rescaling to use one variable

erc-image-inline-rescale:
- nil     : no rescaling
- 'window : rescale, down only, to window size
- <number>: rescale so the height will be equal to <number>
Merge /home/mrb/.emacs.d/el-get/erc-image into fixed-scaling

* /home/mrb/.emacs.d/el-get/erc-image:
  Refactor inline rescaling to use one variable