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.