Add a dynamic index of extensions and descriptions

[lkcampbell]

It is difficult to find an extension of interest in this repository based purely on the author's name and, sometimes, even on the name of the extensions file. Would be useful to have a process that dynamically writes basic information about each extension file (either on a scheduled time or after a file modification) for developer's to use as a reference. Something similar to the data provided by the Inform 7 Installed Extensions window on the Mac: