changes to `mpd_select_colours`

and added some helper functions... I have been working to make my workflow with `rPlotter` more efficient, which meant breaking up `mpd_select_colours` into a few functions. I also added the ability for `extract_colours` to take an object of class "Image", so if you've already loaded an image for some purpose, you don't have to download it again to use `extract_colours`. I also added proper imports for `colorspace` and `picante`, instead of using `require`, which is bad practice. While doing this I used my own package workflow, which uses `Roxygen2` to generate the `NAMESPACE` file. Hopefully this is alright with you. If this messes up how you like to work with `NAMESPACE`, then you should reject this pull request, and I can work out how rewind my changes so only the function changes are in the pull request, leaving the `NAMESPACE` alone.