Add .cssSelector to methods like tagAppendChildren(..., .cssSelector = NULL)

[schloerke]

If the .cssSelector is not NULL, call tagQuery(tag)$find(.cssSelector)$append(...)$allTags().

Do this for other method like tagAppendChildren().

This gives more motivation to export tagInsertChildren() (#225)

[wch]

We'll need to make sure that users are aware that if they do this, it goes through tagQuery(), and is subject to whatever changes tagQuery will make to the object. For example, lists and tagLists will be flattened when this happens. I don't know offhand what other changes tagQuery() will make, but we should make sure they're all documented.

[schloerke]

Should just be the flattening of TAG$children into a single list

[schloerke]

Internal tag name ordering is fixed in #249