Extract annotations based on their color

To address issue jlegewie/zotfile#146

1) convert extracted colors from dRGB (pdf) to HSL
2) cluster colors into groups: dark green, light green, etc. -> green
3) separate annotations by color if *PDFJS.getPDFAnnotations* is called with *useColor* set to *true* (*false* will return the original annotations object without color separation)

I also added some test pdfs that contain annotations with different colors.

Please let me know what you think of my approach :)