excerpt, filetype, image caching

Improvements:

* Now you can create custom excerpts by introducing a "---" or something, that will get parsed to "<hr>" into your text 
* now the available filetypes are loaded automatically
* image caching is now in a seperate class, callable from everywhere

However, i noticed a huge speed drop during development of the last step.
I think this may have to do with the caching of the markdown pictures (they recuire to load the whole config again in the \lorbeer\tools\image), so that could be the reason for long loading times