bug#41766: Make it possible to change regexp to identify and highlight grep matches via customization

[Eli Zaretskii]

> From: Simon Lang <simon.lang@outlook.com>
> Date: Mon, 8 Jun 2020 20:25:06 +0000
> 
> +(defcustom grep-regexp-match "\033\\[0?1;31m\\(.*?\\)\033\\[[0-9]*m"
> +  "Regex definition to identify grep markers to highlight matches.")

Please don't forget the :version tag and the NEWS and the manual
updates.

Thanks.