Adds a write action to the filemgr agent

I needed the possibility of writing small amounts of text to arbitrary files, instead of re-inventing the wheel and  writing an entire custom agent/application I decided it might be suitable for me to modify the filemgr agent.

If it can be useful to somebody else or if you feel it's suitable to include in the official filemgr agent/application. I would be happy to make further modifications as required.