Add GET parameters for changes viewport size.

width
height
Update README.rst
Merge pull request #5 from mozillazg/patch-1

Add GET parameters for changes viewport size
Update README.rst
Fixed width and heigth parameters bug
Fixed TypeError
Make the code shorter
Merge pull request #6 from mozillazg/patch-2

Fix default behaviour if no width/height provided
Fixed an typo issue
Merge pull request #7 from mozillazg/patch-1

Fix an typo issue
Merge pull request #1 from makinacorpus/master

sync with main repo