Add <link rel="next"> (and "prev") tags.

[jameysharp]

These are handy for programmatic tools indexing the comic. Like, say, Comic Rocket :-) or Google (http://googlewebmastercentral.blogspot.com/2011/09/pagination-with-relnext-and-relprev.html).

Note that I have not tested this patch. I think it should be pretty hard to get wrong, though? :-) But then, I haven't used Flask before so I could be missing something important.

[MostAwesomeDude]

Hm. I'll have to double-check that that this actually works right; chrono might not be in scope. Our comics templates are kind of a mess. Also chrono might not always be filled in; we probably only want to set these for comics which aren't the first or last in the archive. As http://www.newrem.com/flavor-text/comics/21/kids says, those comics at the back and front aren't likely to change, at least for heavily-anachronistic series like Flavor Text.