Adds a star to an item.

Optional Params

• channel - Channel to add star to, or channel where the message to add star to was posted (used with timestamp).
• file - File to add star to.
• file_comment - File comment to add star to.
• timestamp - Timestamp of the message to add star to.

Errors the API can return:

• already_starred - The specified item has already been starred by the authenticated user.
• bad_timestamp - Value passed for timestamp was invalid.
• channel_not_found - Channel, private group, or DM specified by channel does not exist
• file_comment_not_found - File comment specified by file_comment does not exist.
• file_not_found - File specified by file does not exist.
• message_not_found - Message specified by channel and timestamp does not exist.
• no_item_specified - file, file_comment, channel and timestamp was not specified.