[ENHANCEMENT] Interface for adding multiple tags at a time

Currently with the new "named tags" design based on a dictionary, you can add or update a tag using its name with:

i = Index(2)
j = settag(i, "site", "1")

and delete a tag using its name with:

k = unsettag(j, "site")
k == i # true

It might be nice to have a syntax for adding multiple tags at once. Currently, settags is used as an internal function for setting all the tags at once, with existing ones getting removed:

i = Index(2)
j = settag(i, "site", "1")
k = settags(j, Dict(["link" => "3"])) # This is currently only defined in `IndexName` but this is for demonstration purposes
hastag(k, "site") # false

so it is meant to be low-level, and I think it is a good name for that.

Maybe we can use the syntax mergetags, i.e.:

i = Index(2)
j = settag(i, "site", "1")
k = mergetags(j, Dict(["link" => "3"]))
gettag(k, "site") == "1" # true
gettag(k, "link") == "3" # true

following the usage of Base.merge on Base.Dict:

julia> merge(Dict("foo" => 0.0, "bar" => 42.0), Dict("baz" => 17, "bar" => 4711))
Dict{String, Float64} with 3 entries:
  "bar" => 4711.0
  "baz" => 17.0
  "foo" => 0.0

The issue names no files or tests; start by locating the Index tag methods and the existing settags implementation. Compare the proposed merge behavior with current settag, unsettag, and settags usage, then confirm the public API and collision semantics with maintainers. Done means an agreed interface and tests covering preservation and replacement of tags.