Fix TocJS index file extension

Commit 99822a8337e37d5df7de6573e97bdca08cbbd0a9 introduced a bug that caused the
index file of TocJS output to be named "index${out.ext}" instead of "index.html"
by default.

Also, I simplified things a bit since there's no reason to do something like
this:

  <condition property="out.ext" value=".html">
      <not>
          <isset property="out.ext" />
      </not>
  </condition>

Ant properties are immutable, so you can just set them normally:

  <property name="out.ext" value=".html"/>

If the user sets the property before this, Ant will simply ignore this
<property> element.
Merge pull request #1554 from emth/tocjs-ext

Fix TocJS index file extension initialization.