Some new color commands

Hi Ale!

The complete list of commands is: 
- Blend Color - [video](https://www.dropbox.com/s/hlibjm1zwr92gqr/color-blend.mov)
- Copy Fill Color Blended
- Copy Fill Color to RGBA
- Darken
- Lighten
- Saturate
- Desaturate
- Shift Hue

Color manipulation it's a pain. So, I used [color-js](https://github.com/brehaut/color-js) but I'm not sure if you agree with inserting third-party libs into orange commands. 

The other thing I'm not sure is the path for color.js library, is currently located at `Commands/color.js` but, what about moving it to a `vendor/` directory? This is related with the above one :)