Add support for track numbering in ID3 TRCK style (NN/MM).

[jamesyoungman]

This fixes https://github.com/TianyiShi2001/audiotags/issues/5

Example FLAC metadata using this style:

comment[0]: album=Chosen Lords
comment[1]: discid=820d4b0a
comment[2]: title=Reunion 2
comment[3]: artist=Aphex Twin
comment[4]: tracknumber=2/10

[pinkforest]

We just re-merged audiotags2 into audiotags https://github.com/TianyiShi2001/audiotags/pull/18

[nVitius]

@pinkforest This PR is still relevant to the open issue. Can we get this merged?

I also added another comment on the linked issue (#5) expanding on why this is a useful change.