Get all bookmarks in the users's account in JSON format

Without passing the 'format=json' with the request to the Pinboard API,
API_POSTS_ALL returns every bookmark associated with the users's account in XML
format. Since the Alfred Workflow by Jaemok Jeong for Pinboard expects the file
to be in JSON, we need to specify the format to avoid breaking the workflow
when downloading all bookmarks with this script.