Cleanup improvements

Added option 'cleanupdefs' (default: false); when set to true no cleaup is done on elements inside the <defs> element. This can be useful when one does not want to remove style-attributes for elements inside <defs> elements. For example, on usingdef.svg (blob/master/test/fixtures/usingdef.svg) one may not want to remove the style attribute (style="stop-color:rgb(255,255,0);stop-opacity:1") which sets the style for the linearGradient. For reasons mentioned in #37, one may want to remove all other style attributes.

Changed option 'cleanup'; it now accepts an array of attributes. For backwards compatibility cleanup === true equals ['style']. The attributes in the array are removed from all elements. This can be useful for example when one want to set the fill of a drawing using CSS.