Icon colours

[IvanSanchez]

This implements a canvas-based hue rotation of the default icon for this plugin. So instead of blue markers, the user can have other colours.

Things to do before merging the code:

• Investigate the possibility of using CSS colours and calculating the HSL shift(s) needed
• Implement white & black markers, maybe grey too
• Write docs about L.Icon.Glyph.getColorIconUrl(color)
• Make the demo-iconcolor.html example prettier, fix the text

[IvanSanchez]

@george-silva Help wanted with the missing bits

[george-silva]

@IvanSanchez cool. I surely can help with the last two 👍
Give me some time to dig in and I'll try first two.