Atrament v1.2.0

This release removes mentions of the opacity option, which was removed in v1.0.0. I believe the option wasn't very useful, because the way Atrament draws the strokes is via many composite curves, which may overlap. When using the globalAlpha setting, this means the resulting stroke doesn't have uniform opacity, but is rather composed of a number of semi-transparent layers.

Thanks @alireza23 for pointing out the broken opacity slider in the demo (#52)