Allow updating of comments that weren't imported with feeds_comment_processor

Currently, there is no good way to update comments that were not imported with feeds_comment_processor. If you are trying to update existing comments that weren't imported with feeds_comment_processor, new comments will always be created.

The attached smallish patch should fix this.

I will be testing this patch on the site I work on for my day job, but would appreciate others testing too. The only catch is you have to specify some way for feeds_comment_processor to know which comment to update.

You can do this in 2 ways:
1. Include the new "Comment ID" target in your mappings, and be sure to set it as a unique target. If you want a comment to be inserted instead of updated, you MUST leave the cid field blank.
2. Override existingItemId() in your processor

Admittedly I've only tested #2 so I will need to test #1 before I commit.