Implement filters info

[Euklios]

This parses the output for ffmpeg -filters and creates corresponding objects.
Created analogue to the codecs() and formats() methods.

[bramp]

Thanks. I appreciate how much effort you are putting it to keep this consistent with the existing code styles.

[Euklios]

Thanks. I appreciate how much effort you are putting it to keep this consistent with the existing code styles.

No worries, I'm currently maintaining four forks of this library (and many, many branches).
Sticking to the original version is about the only reason I can still keep them more or less up to date.

And as I said before, this library is a heaven-sent to me.
The project I'm using it in is closing in on 100k LOC, and having something to take away a bit of the complexity ffmpeg poses is invaluable.

[bramp]

Thanks. Also, while I don't put much effort into this project, I'm happy to try and get all your pull requests in, so you don't need to manage so many external versions.