Integrate VexTab with markdown-it-music

[shadanan]

This PR enables markdown-it-music to render vextab notation using:

:::vextab
options space=20

tabstave
  notation=true
  key=A time=4/4

  notes :q =|: (5/2.5/3.7/4) :8 7-5h6/3 ^3^ 5h6-7/5 ^3^ :q 7V/4 |
  notes :8 t12p7/4 s5s3/4 :8 3s:16:5-7/5 :h p5/4
  text :w, |#segno, ,|, :hd, , #tr

options space=25

tabstave
  notation=true

  notes :q (5/4.5/5) (7/4.7/5)s(5/4.5/5) ^3^
  notes :8 7-5/4 $.a./b.$ (5/4.5/5)h(7/5) =:|
  notes :8 (12/5.12/4)ds(5/5.5/4)u 3b4/5
  notes :h (5V/6.5/4.6/3.7/2) $.italic.let ring$ =|=

  text :h, ,.font=Times-12-italic, D.S. al coda, |#coda
  text :h, ,.-1, .font=Arial-14-bold,A13
  text ++, .23, #f
:::

Which renders:

Fixes #15