Implement req_cookie_file()

[hadley]

Fixes #223

[hadley]

@mgirlich what do you think of the name?

[jonthegeek]

This looks super useful! Some thoughts on the name:

• req_cookie_file() makes it feel like the thing I care about is the file, moreso than the reuse of the cookies.
• Would it make sense to align it with the cache_disk and cache_key options from the _auth_ functions? It's not QUITE the same thing but feels similar.

I guess the difference here is that you could conceivably supply a file from some other source, whereas caching is more internal and automatic.

So after talking it out... I can't think of anything that makes more sense to me than req_cookie_file().

[mgirlich]

Basically, req_cookie_file() is fine but maybe something like req_cookie_preserve() would be easier to remember.

[hadley]

Oh I like req_cookie_preserve().

[hadley]

@jonthegeek the implementation here is pretty limited by what curl provides; it has to be written out to disk.