Adds atrim filter.

[jcordial]

I had been trying to trim audio files using the trim filter. After reading a bunch of ffmpeg documentation, it turns out you don't trim audio files with trim. I needed to use atrim, which hadn't been added to the api. Sooooo.... I added it.

I'm not 100% sure what the best testing strategy is for adding new filters, they don't seem to be tested directly. If you need me to add a test, let me know.

[jcordial]

Actually, I reread the documentation - I see there's the generic "filter" method for things like this. Feel free to close this PR if you don't want to support this particular filter directly.