Reformatted and gjslinted example-screenshot.js.
Restrict progressbar to fixed width.

As more and more examples will be processed with this script, the
progresbar would grow way too long. This commit makes the width of
the bar fixed. Every character in the bar now equals 5% progress,
regardless of the number of examples.
Merge pull request #379 from marcjansen/progressbar

Restrict width of progressbar in example-screenshot.js