Allow direct attributes giving on a field

[aminya]

It helps to avoid creating new struct if the field is just a text and some attributes.
Attributes can be given in a Tuple with the xml name of them:

@aml mutable struct tr "~"
  th::String, "~", missing, ("class", "style")
end

Two commas were used because the first one is the place holder for the value checking function.

For accessing the Dict after creation, one should be able to do this:

tr.th # regular content
tr.th["class"] # attribute

[aminya]

Thinking more about this:

• This is just defining a new type in 1 line, while it can be split easily (not sure how much value we get from this quicker syntax)

• What should be given in the Tuple:

• just giving the xml name? and assuming only strings?

("class", "style")

• giving all the argument information? -> similar to just defining separate types

(x::String, att"class", y::Int64, att"style")

• How can they instantiate a type with attributes?

tr( th = th("good", class = "this", style = "that"))

• How can they access the attributes?

tr.th # regular content
tr.th["class"] # attribute