Implement loose cooking parsing

[Sebmaster]

This enables correct execution of code like document.cookie = 'foo'.

Blocked on either request/request#1811 or salesforce/tough-cookie#56.

[domenic]

LGTM except for the commit message (s/cooking/cookie), so once one of the prereqs gets merged feel free to merge.

[Sebmaster]

@domenic Added loose mode to the constructor too now. The problem right now is (without request merging the PR), if a user of jsdom provides a cookie jar without loose mode, we'll handle malformatted cookies in document.cookie differently, than if we get the cookie from a resource request, which kind of sucks. We could set jar.enableLooseMode, but I'm not sure if it's considered public API.

The alternative would be to provide it just to the jars we create ourselves, so the behaviour is consistent across doc.cookie and requests, but not browser-like (malformed cookies are refused), unless users properly create the jar.

[Sebmaster]

Modified to only set loose mode on jars we create ourselves. If a user passes us a jar without loose mode, we'll behave strict and refuse malformed cookies.

[domenic]

LGTM!