Handler.Feed: colorize package version diff

[develop7]

A follow-up to #247 (couldn't test myself again due to to yesodweb/yesod#1471)

[snoyberg]

Thanks, merged! Unfortunately this likely won't do much in most RSS readers, which will ignore the style attributes. I made a minor modifications to use the del and ins tags instead of span, just in case some readers style those more nicely.

[develop7]

@snoyberg thanks! del and ins make more sense indeed, as clients have a built-in styles for them