Add numeric color values to 07-customizing-git.

In `en/07-customizing-git/01-chapter7.markdown`, add an explanation that users can specify numeric color values instead of string names when colorizing `git` output.
@daftano Not sure if this changes your opinion, but they're backticks.

@jnavila I thought that displaying each of the values as inline code would give them an appropriate visual distinction, because they're what would appear directly in your `.gitconfig` (not unlike how I'd write a command line flag as `--flag` instead of --flag. Anyway, unless you both still disagree, I'll gladly remove them in another commit and re-submit my pull request: my patch was really only intended to add information about numeric color values, so the backticks are secondary.