Pm2 accessory type

Adds the ability to set `:accessory_type` in a `PM::TableViewCell`

Default is none, but options are:

```ruby
{
  none:                 UITableViewCellAccessoryNone,
  disclosure_indicator: UITableViewCellAccessoryDisclosureIndicator,
  disclosure_button:    UITableViewCellAccessoryDetailDisclosureButton,
  checkmark:            UITableViewCellAccessoryCheckmark,
  detail_button:        UITableViewCellAccessoryDetailButton
}
```

Fixes #482