Add the MML command of the portamento/slur start and end.

[Shaw02]

Command draft

Slur

• Son
• Soff

Example
cdef Son gab>c Soff dc<ba Son gfed Soff c1
is equal
cdef g&a&b&>c dc<ba g&f&e&d c1

Portamento

• Pon
• Poff

example
l4 Pon8 c g e >c Poff
is equal
l4 {cg}8&g8& {ge}8&e8& {e>c}8&c8& c4