Add keyboard support

[captainbrosset]

It would be nice to be able to tab/shift-tab between color stops.
With a color stop focused, the arrow keys could be used to move the stop back and forth on the gradient line. The alt and shift modifiers could be used to move slow and fast.
Enter could add a new color stop.
Delete could remove the focused stop.
The angle should also be focusable.
It should be easy to tab through the whole app, from the input field to the angle, to the stops, back to the input field.