[feature] toggle background color on alternating rows

Nobody has claimed this yet.

Alternating the background color (optionally) of each row can make things much more readable, especially when the tables are particularly tall or wide.

proposal:

1. alternating colors are off by default
2. default background color when turned on.
3. optionally specify an RGB colors to use for the background color.
- `alternating_colors => [[r,g,b],[r,g,b]]`
- no limit to the number of alternating colors. the system would just choose the next color in the list and then jump back to the starting color when it runs out. `Nil` is a valid option and would just result in _no_ background color. Users could specify something like `alternating_colors=>PrettierTable.alternating_rows` if they wanted the defaults, and that would just be `[Nil, [r,g,b]]` where the rgb defined some dark grey.

No files or tests are named. Start by locating the table-rendering and configuration entry points, then define the behavior for disabled, default, RGB-list, Nil, and wraparound cases. Done means the feature is optional, supports the proposed color choices, and has coverage for the specified behavior.

Written by the indexing model from the issue text.