Changes the screenshots-scale in favor of the screenshots-size option
which is more flexible.
Promise-based viewportSize setting to support frameworks that rely on the
onresize event.
Removes the gif choice for image format as it's not working.
Documents the --screenshots option.
Fixes#9.
Adds the --screenshots, --screenshots-directory, --screenshots-scale and
--screenshots-format options.
If the --screenshots flag option is set, each slide is exported as an
image, whose format is --screenshots-format, written in the
--screenshots-directory, and this for each --screenshots-scale.
Changes the viewport size option to follow the <width>x<height> notation.
