add modularity

[doctorcolossus]

Hello, I just came across this project and my initial impression is that the predefined filters look potentially useful, but are quite fixed. What if I want to do something else? I think it would open up a lot of possibilities if weffe defined a framework for filter plugins from the greater community, and the existing plugins were showcased more as examples of how to do this. Right now it is monolithic, in that all of the code is in one huge file, the filters are fixed, and it's not immediately clear how or where to add a new one, where to begin with that, or what would be required to do so. Of course the code is all there and I or anyone else could figure it out given enough time, but implementing some kind of modular framework to streamline the process could lead to a richer collection of filters.