Added feature to set bg_color using -B flag.

[Demkeys]

Hello, I have added a feature to allow the user to specify a background color using the -B flag, similar to how they would specify a color with the -C flag. There is no default value for -B so if user doesn't specify it, it never gets set.

Implementation:

• Added a bg_color field of type Option to the Cli struct.
• Added rain_bg_color() to get background color in Option<u8,u8,u8> form.
• bg_color being Option allows us to ignore bg_color if user hasn't specified the -B flag. If bg_color is passed in through CLI arguments, then SetBackgroundColor() sets the background color to that color.
• In setup_terminal() I wasn't sure if I could put the logic into the same execute!() call so I made a separate one to call SetBackgroundColor().

Notes:
One small caveat is that background color won't fully take effect until the program has drawn characters on all cells visible on the screen, so for the first few seconds of the runtime you'll see something like this:

But that only lasts for a few seconds, and eventually the background color fills the entire screen.

[Demkeys]

Accidentally closed. Reopened it.

[cowboy8625]

Hey I like this! I've been thinking about doing this my self! Thanks! :)

[cowboy8625]

Just two small comments, but over all looks good. I still need to test.

[Demkeys]

Ok done. Also thanks for the tip about Clear()!

[cowboy8625]

Awesome! Thanks for your interest in the project!