Add a trigger input to the line unit generators

[colinbdclark]

Currently, there's no way to trigger a flock.ugen.line or flock.ugen.xLine to restart once it's finished, or to delay its start until a particular time.

We should add a new trigger input to them both, defaulting to 1.0 so the current behaviour is retained, which allows lines to be triggered by a standard trigger signal.

Of course this is possible now by using a line-shaped envelope with flock.ugen.envGen instead, but the line unit generators are so simple and easy to use that it seems justified to add this feature to them as well.