Clarify bookmark creation

[SeBBBe]

Description

In the SDK, CreateBookmark accepts a custom BookmarksProperties object that contains, among other things, the modified fields and current sheet. In enigma-go, the equivalent method only accepts a generic NxInfo and NxMetaDef. There is a CreateBookmarkRaw method that accepts an interface, so it's possible to pass properties dynamically.

Create a helper struct to enable full functionality using CreateBookmark or document how to use CreateBookmarkRaw with an example.

[SeBBBe]

I have gotten it working using code as follows:

`
// Construct the required fields dynamically
metaDef := make(map[string]interface{})
metaDef["description"] = settings.Description
metaDef["title"] = settings.Title

nxInfo := enigma.NxInfo{
	Id:   settings.Id,
	Type: "bookmark",
}

// Mirrors the fields in the SDK
props := make(map[string]interface{})
props["sheetId"] = sheet.ID
props["selectionFields"] = fields
props["creationDate"] = time.Now().Format("01/02/06 ") // US short date format
props["qMetaDef"] = metaDef
props["qInfo"] = nxInfo`