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

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.