Added plugin mode to text panel

A bit of background:
I'm currently working on an open source monitoring system for java based (web) applications.
* Backend: Graphite
* Frontend: Grafana
* Custom collector agent, including a profiler

My problem is, that I have requirements that are too specific to include them in grafana like displaying call stacks collected by the profiler. So I added a plugin mode to the text panel that solves this problem.

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The plugin mode offers two textareas. The first one can be used to implement additional controllers, directives and filters. In the second one you can enter angular.js HTML templates that reference the new components. This way it is easy to augment grafana with specific functionality within a dashboard itself and without maintaining an own fork of the project.

I hope you find this useful