Add header images and background colors. Fixes #52

[atodorov]

See my comment at
#52 (comment)
for implementation specific details

[duilio]

I see you have listed the options in the same order of priority given in the code. Can you write just a line before to make it explicit in the doc?

Also, wouldn't it be better to actually give more priority to the article's properties instead of cover -> color? Something like:

1. article's header_cover property
2. article's header_color property
3. global HEADER_COVER
4. global HEADER_COLOR

[atodorov]

ping, any updates here? This can still be merged directly.