- implement credits metabox to assign credits to each post (post, page, custom post) defined during plugin configuration
  - retrieve default values for new post, page or custom post
  - if not set a value define the default values only for new post
  - for old post if no value is defined set it to 0
  - negative value are not allowed (they will be saved as zero)
  - save data as post_meta (credits-coins-post-value)