Add 256 colour support

[Astrid-Lauen]

This pull request adds 256 colour support that can be enabled by compiling with cargo run --features 256colour. 256 colour support is useful for terminals without 24 bit colour support such as the macOS terminal.

This pull request also refactors part of the code by replacing the static methods Colour::ansi_text_colour_string and Colour::ansi_bg_colour_string with instance methods to make code cleaner.

[Astrid-Lauen]

Here is what the game looks like with 24 bit colour (first image) and 256 colour (second image) on a terminal without 24 bit colour support:

[dmitmel]

Whoops, I've approved the wrong commit

[Hopson97]

Looks good, thanks!