Add "Launch" option to installer

As with many other installers, it would be nice to have a checkbox to allow the option of "Launch Sublime Text 2" on the installation complete page (I'd prefer it default to be checked as well, but that's up to you).

For the record, I'm on the dev channel, so I suppose it may be included in the installers for the official builds. A minor change, but I'd find it very useful.