Add Converter for doing Advanced Shit™ in your CSV conversion

I'm writing this library to replace all the bits of the pandas python package we use for a little glue app we have at work (because replacing small python things with small go things is a good thing, right?)

The first cut of the API covers 7 of the 8 table dumps that we need to do, now I'm rounding out the API for some of the fancier stuff we do.

Would appreciate feedback on the API as I'm designing it, and also because I appear to be half asleep this sunday I must be doing something stupid, as my approach for overriding headers is not working.

If someone could point out the specific kind of dumb I'm being I'd be quite grateful.

ping @nf @davecheney @lox @mtibben if you've got a couple of spare minutes