Add environment configuration option to set terminal background color

[igorshubovych]

I use dark terminal. It is a pain to write every time --bg "dark".
It should be simple to read some environment variable (or any other type of global configuration file, e.g. ~/.ccat file) and define it globally. Then command line argument can just overwrite the global configuration, if present.

P.S. I want to add that light scheme looks ok on dark terminal, but i use dark terminal with transparency and only then it is unreadable. As it is unreadable for small portion of users, i do not think it is critical.

[owenthereal]

👍 I plan to support this!

[owenthereal]

@igorshubovych The color scheme is not optimized. Feel free to help improve this. I also plan to make the color scheme customizable so that you could define yours. I pre-populate a bunch of color codes here.

[owenthereal]

Actually just another thought. How about alias ccat="ccat --bg=dark"?

[igorshubovych]

Good idea

[owenthereal]

@igorshubovych I made a PR to provide options to set color values: #27