Adding linear gradient

add a color gradient with bgcolormid/bgcolorend and fgcolormid/fgcolorend  parameter that correspond to the transitional color and gradient end color.

example of set : <input class="knob" sata-step="1" data-min="0" data-max="150" value="0"  data-inputcolor="#ff0000" data-fgcolor="#ff0000" data-fgcolormid="#800000" data-fgcolorend="#000000" data-thickness="0.2" data-bgcolor="#000000" data-bgcolormid="#008000" data-bgcolorend="#00ff00">