descriptive docs variable naming

Created by: m5o

• rename .btn-bd-yellow to .btn-bd-download
• rename .btn-bd-purple to .btn-bd-primary
• sort state vars
  • info, warning, danger
• generate $bd-purple-light color
  • found match with http://razorltd.github.io/sasscolourfunctioncalculator/
• allow overide of docs variables