5.7.0 Release: attribute namespace, docstring, trailing =

- Fix #19: A common typo when writing HLisp is to set attributes on
  elements like this:

      (div :id="foobar" :class="main fancy" ...).

  This causes a runtime error, though, because internally Hoplon will
  try to do something like:

      element.setAttribute("id=", "foobar"),

  but element attribute names may not contain the = character. This
  commit simply removes the trailing = character from the keyword name
  when necessary.

- Fix #23: Preserve namespaces of element attribute keywords. For
  example:

      (div :foo.bar/baz "100%" ...)

  The previous behavior was to discard the namespace. This commit
  introduces an issue with the HTML syntax representation, because the
  slash character (/) isn't allowed to appear in attribute names. So in
  this case use the dot character (.) instead of slash, like this:

      <div foo.bar.baz="100%">...</div>

  This does of course mean that the name part of the keyword must not
  contain any dots.

- Fix #24: Add support for docstring, etc. (all the things supported by
  defn except for multiple arities) in defelem.