Script asks for page range, and can export single pages

Building single-page export into the script avoids possible user error when exporting single pages by hand. (Mainly by setting the wrong page range, and not realising until it's too late.)
Added 'Web PDF', renamed Trinity export script

Web PDF.scpt opens and exports a selected set of InDesign files using the
smallest size setting, intended for use for the Star's online .pdf edition
after being combined into a single file using Acrobat.

It uses each file's page count to determine which pages to export, similar
to the Trinity .pdf exporter.