Use cookie to remember if user want to subscribe

[omry]

I modified my copy of stc so that it will use a cookie to remember if the user want to subscribe to comments. the logic is as follows: if there is a cookie, use it to determine if to make check the box, otherwise use the default value from the options. on the surface it seems to work correctly (didn't give it a real test yet).

the diff is attached.

[omry]

One minor fix that will prevent possible PHP warnings in some cases:

[markjaquith]

This has been implemented. Thanks!