Add extension points

[fxnn]

I want to make gone an application that fits many purposes, yet as simple as possible. Therefore, we need extension points (Open Closed Principle). This could include the possibility to call completely new functionality from the UI. Such a concept would greatly complement with the customizeable UI templates.

Stories

1. An administrator wants to incorporate version control into his gone installation. Now, without modifying the gone sourcecode, he makes gone execute git commit -a everytime a file is saved.
2. An administrator wants to offer every user of his gone installation the function of publishing all files inside the content root onto a public web server. Therefore, he modifies the UI templates, adding a special link. That link points to a URL which makes gone call a script that publishes the files. The gone sourcecode wasn't changed for this.
3. An administrator wants to offer a function where each user can see the VCS history of the current resource. Therefore, he modifies the UI templates, adding a special link. That link adds something to the resources URL, which makes gone call a script. That script writes the VCS history to stdout, being displayed by gone. The gone sourcecode wasn't changed for this.