Extend `c.Add` to include position and extend both `c.Add` and `c.AddID` to include relative position...

[iconoclasthero]

// Add adds the file/directory uri to playlist. Directories add recursively.
func (c *Client) Add(uri string) error {
	return c.Command("add %s", uri).OK()
}

// AddID adds the file/directory uri to playlist and returns the identity
// id of the song added. If pos is positive, the song is added to position
// pos.
func (c *Client) AddID(uri string, pos int) (int, error) {
	var cmd *Command
	if pos >= 0 {
		cmd = c.Command("addid %s %d", uri, pos)
	} else {
		cmd = c.Command("addid %s", uri)
	}
	attrs, err := cmd.Attrs()
	if err != nil {
		return -1, err
	}
	tok, ok := attrs["Id"]
	if !ok {
		return -1, textproto.ProtocolError("addid did not return Id")
	}
	return strconv.Atoi(tok)
}

The protocol accepts both an absolute and a relative position for both add and addid. Both are unavailable in c.Add and the latter is unavailable in c.Addid.

add {URI} [POSITION]
Adds the file URI to the playlist (directories add recursively). URI can also be a single file.

The position parameter is the same as in addid. 10

Clients that are connected via local socket may add arbitrary local files (URI is an absolute path). Example:

add "/home/foo/Music/bar.ogg"

addid {URI} [POSITION]
Adds a song to the playlist (non-recursive) and returns the song id. URI is always a single file or URL. For example:

addid "foo.mp3"
Id: 999
OK

If the second parameter is given, then the song is inserted at the specified position. If the parameter starts with + or -, then it is relative to the current song 8; e.g. +0 inserts right after the current song and -0 inserts right before the current song (i.e. zero songs between the current song and the newly added song).

The relative-to-current positional facility would be nice to have but as I pointed out in #90 there's no way to add recursively with addid so there's therefore no way to add recursively at a position in gompd which is a significant limitation in utility.

[iconoclasthero]

Since there's a protocol version limitation here, the documentation will need to reflect that as shown in the protocol documentation.