New Chant extension

[GiovanniSalmeri]

Another little extension: Chant. It does exactely the same thing as Music, but for Gregorian chant. It uses the excellent library exsurge (I used an old version, beacuse the new forked ones exhibit various glitches in webkit/Chrome-based browsers: no time now to investigate if the error is on my part!). And this is the Christmas chant shown as example: Puer natus est nobis, magnificently sung by a female chorus!