Fix audio segment export bigger 4gb

[inatuwe]

This pull request adds the possibility to export an AudioSegment, which is bigger than 4 GB to any compressed format. It does not allow to export the AudioSegment directly to a WAV file bigger than 4 GB.
Apparently it's not even allowed from the specification of the WAV file to be bigger than 4 GB.
The strategy for the compressed formats is to split the audio segment into Wave Files smaller 4 GB and then to combine them with FFMPEG.
Note:
I also added a test with an audio segment > 4 GB, which is taking quite some time. On my machine (Lenovo P50), it takes 26 seconds.
Additionally on the infrastructure, it's impossible to run it due to memory limitation:

ERROR: test_audio_segment_export_bigger_than_4gb (main.AudioSegmentTests)
Traceback (most recent call last):
File "test/test.py", line 1104, in test_audio_segment_export_bigger_than_4gb
original_segment += original_segment
File "c:\projects\pydub\pydub\audio_segment.py", line 364, in add
return self.append(arg, crossfade=0)
File "c:\projects\pydub\pydub\audio_segment.py", line 1280, in append
return seg1._spawn(seg1._data + seg2._data)
MemoryError

=> This test is there, it can be used to verify the issue and the fix, but I have to disable it.

[cvillela]

Any help on this? I am facing the exact same issue.....

[aedocw]

Would love to see this merged, it would help with an issue some folks hit when making audiobooks.