Consider removing children keyword argument and __call__ method on Tag

[cpsievert]

The original inspiration for having the two is to provide a way for attributes to appear before children, e.g.:

from htmltools import *
div(
  class = "foo", id = "foo",
  children = [
     ...children
  ]
)
# or
div(class = "foo", id = "foo")(
    ...children
)

But after #13, we'll be able to do:

from htmltools import *
div(
    dict(class = "foo", id = "foo"),
    ...children
)