title: With Colors.jl, how to change `Color`'s alpha transparency value?
body: <p>If I have a Colors.jl <code>Color</code>, such as <code>HSLA(200,0.9,0.8,0.8)</code>, <code>HSL(200,0.9,0.8,0.8)</code>, or <code>RGB(200,100,100)</code>, is there a generic way to return a version of the color with a specified alpha transparency value (and promote the type if the input doesn't have an alpha value)?</p>
  <p>E.g. something like <code>set_alpha(in_color,alpha=0.9)</code></p>