#59 stretchlast should be float not boolean

[Christopher Roland Ritson]

%%stretchlast is described in abcm2ps as being a float, but the easyABC says it is boolean in Help -> ABC Fields and Command Reference -> spacing

Stretch the last music line of a tune when it lacks less than the float fraction of the page width.
float range is 0.0 to 1.0.

[Frederic Aupepin]

reference field is updated in latest pull request on github. This change is part of the update.