Add size_threshold_ignore to HTML, to ignore specific pages

[mortenpi]

Suggested by @odow in #2205 (comment), to have a bit more fine grained way to ignore some pages going over the size threshold.

Behavior-wise, I would suggest that the ignored pages still warn if they go over the size_threshold_warn limit, as this is supposed to be a temporary solution. So effectively it sets size_threshold = Inf for those pages.

[mortenpi]

Another option would be to have size_threshold_override :: Vector{Pair{String, Int}} = [page => threshold, ...], to set a special threshold for some pages. You could still do size_threshold_override = ["foo/bar.md" => nothing] to exclude the page completely.