Stream selectors, `.map` operator (audio support)

Created by: depau

Implements stream selection and mapping multiple streams to one output.

Example:

i = ffmpeg.input("video_with_sound.mp4")
audio = i[:"a"].filter_("aecho", 0.8, 0.9, 1000, 0.3)
video = i[:"v"].hflip().vflip()
single_output = ffmpeg.output(video, audio, "my_output.mp4")
# or
single_output = video.output("my_output.mp4")
single_output = single_output.map(audio)

ffmpeg -i video_with_sound.mp4
-filter_complex
'[0:v]hflip[s0];[s0]vflip[s1];[0:a]aecho=0.8:0.9:1000:0.3[s2]'
-map '[s1]' -map '[s2]' my_output.mp4

Closes #26 (closed) and #42 (closed)