Support modern-style elisions (underties)

[lbssousa]

Gregorio already supports ancient-style elisions through <e></e> tags, but it would be useful to support modern-style elisions (underties) as well. It would be valuable for neogregorian compositions in vernacular language.

I currently use \itie command (from metre package) as follows:

{<v>\itie{a e}</v>}(gh) % underties vowels a and e