Implement Mp3Compression transform

[iver56]

It's not really ideal, since it writes to disk, but at least it is functional.

Close #37

[codecov]

Codecov Report

Merging #38 into master will decrease coverage by 1.97%.
The diff coverage is 89.18%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage   99.11%   97.13%   -1.98%     
==========================================
  Files           4        4              
  Lines         450      523      +73     
==========================================
+ Hits          446      508      +62     
- Misses          4       15      +11     

Impacted Files	Coverage Δ
audiomentations/core/utils.py	94.11% <75.00%> (-2.55%)	⬇️
audiomentations/augmentations/transforms.py	97.45% <90.00%> (-2.00%)	⬇️
audiomentations/core/transforms_interface.py	96.87% <0.00%> (-3.13%)	⬇️

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[iver56]

This approach may be better:
facebookresearch/demucs@7157c08#diff-1c77eb608c381bd9ce78849390fd6311R82

[iver56]

Actually, I think we should support both backends. Pros/cons:

lameenc:

Pros: More flexible parameters. Faster than pydub/ffmpeg.

Cons: Introduces latency in the audio

pydub/ffmpeg

Pros: Does not introduce latency in the audio

Cons: Slower than lameenc