Mechanize intercept

I don't want an immediate merge of this, it will quickly need a rebase anyway. Just wanted to spark discussion of what to do about mechanize.

Actually I'm on the fence about bothering to support mechanize at all in the post-Python3 branch, since these days it's a bit unpleasant to use and support (you will note some ugly conditionals/skips in the test and a couple asserts at the top of the new intercept module and I haven't thought yet how I am going to let 'make doctest' run smoothly). But it's pretty easy to get a basic version going, so here it is.

Credit where it's due, this is somehow based on the old mechanize intercept module from the Python2-only wsgi_intercept.

Oh, I forgot the docs/example snippet, but I can make and push one fairly easily if it's needed