Added digest option to PinboardLogger.

I prefer my individual saved pinboard bookmarks to be individual entries in my Day One.  If and only if you set the pinboard_digest option to false (default is true), then these modifications will parse the new bookmarks into individual Day One entries each timestamped with the time the bookmark was created.  Otherwise, if pinboard_digest is true the plugin will run as it did before.
Merge pull request #186 from scrooks/pinboarddigest

Added digest option to PinboardLogger.