Promote use of highlight class in Readme

[GaryJones]

To support a basic install-and-go, we currently add the only style not in the GitHub default style sheet, that of highlighted lines, as a style attribute with a background-color.

We also add a class however, and developers should be made aware, via the README.md file, that they could add something like:

.pre-line-highlight {
    background: #ffc;
}

...to their theme style sheet and:

add_filter( 'blazersix_gist_oembed_highlight_color', '__return_false' );

...to their theme functions.php to avoid that style attribute, and potentially pick a highlight colour that suits their theme.

[GaryJones]

Fixed by d71e4531e59b6bb9b47a308116781f8c43eb1e16.