Add the ability to add diagonal goals

[DavidBennettPIO]

I was is the need of a diagonal goal/target so I added it in.

This is useful for future projections using only the current information available.

For example this is a screenshot of this PR in action, it shows that were about on target, a bit over now but will be a bit under later on.

This works by taking an array of arrays in the goals option ie the obove one looked like:

var feed_wedge = new Morris.Bar({
    element: 'feed-wedge',
    ...
    goals: [[3200, 1800]],
    ...
  });

So similar to the events option, but it draws a line and not a rectangle.