title: Darken or lighten a color in matplotlib
body: <p>Say I have a color in Matplotlib.  Maybe it's a string (<code>'k'</code>) or an rgb tuple (<code>(0.5, 0.1, 0.8)</code>) or even some hex (<code>#05FA2B</code>).  Is there a command / convenience function in Matplotlib that would allow me to darken (or lighten) that color.  </p>
  
  <p>I.e. is there <code>matplotlib.pyplot.darken(c, 0.1)</code> or something like that?  I guess what I'm hoping for is something that, behind the scenes, would take a color, convert it to HSL, then either multiply the L value by some given factor (flooring at 0 and capping at 1) or explicitly set the L value to a given value and return the modified color.</p>