Give ld-export-metadata support for writing Audacity labels files

[atsampson]

CC @Gamnn.

Move the code to extract chapters/stopcodes from the FFmetadata writer into the library, and add a new writer for Audacity labels files.

Fixes #751.

[Gamnn]

Like --ffmpeg-metadata, it would be good if it ignores single-frame chapter changes due to dropouts in the vbi.

Frame 44644 during chapter registered as chapter 12, leaving a gap between chapter 2 and 3.

0.000000    254.354100  Chapter 0
254.354100  887.686800  Chapter 1
887.686800  1489.588100 Chapter 2
1928.693433 2091.923167 Chapter 3
2091.923167 2437.601833 Chapter 4
2437.601833 2632.663367 Chapter 5

[atsampson]

Fixed now - thanks!