command line parameter to limit the final resolution

[sassman]

in order to change the final gif / video resolution (sometimes full retina is a bit overdone) 2 optional parameters should be introduced:

1. a --max-height | h
2. a --max-width | w
3. both should act as upper limits and not be taken precise
4. the aspect ratio of the original format should be kept (pretty much like convert -resize Wx for example works

suggested by

@Hoverbear