Add a simple script to update the version in `index.html`

As we already require `node` to pack the Javascript and CSS, it makes
sense to use `node` for this script, too.

Example usage:

	node bump-version.js '1.9.5 Update 1' 20150115

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
bump-version: introduce `--auto` option

Calling `node bump-version.js --auto` will now update the version
automatically to the latest uploaded release.

This feature took way too much time to develop, but it was a total lot of
fun and this developer practiced his Javascript a bit, so all is good.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>