Implement --background-color

[ISSOtm]

To do:

• Allow transparent as a color
• Allow +color (e.g. +#f0f or +transparent) to not warn when the background color is used within kept tiles (not necessary for the initial feature)
• Update the man page
• Add tests

Fixes #1484

[ISSOtm]

Yep, the logic seems good to me, so I have only a few nitpicks. Thank you for finishing my half-done work :)