Add onRender() method to tag objects

[cpsievert]

Which should remove the need to rely so heavily on "black box" tagFunction()s. This way we at least would have a chance at being able to select "dynamic" tags based on their contents pre-render (related to #208).

We should support both of these interfaces? And have the 2nd version implemented with the 1st?

div(
  span(a("foo")),
  .onRender = function(x) {
    tagQuery(x)$find("a")$asTags()
  }
)

onRender(
  span(a("foo")),
  function(x) {
     tagQuery(x)$find("a")$asTags()
  }
)

[cpsievert]

Will be closed by #215