Fix edge case bugs for displayed equations

- The regex test for a displayed equations block at the begining of a
  file was broken by a previous fix for the regex test for an inline
  equation at the beginning of the file (which added a space to the
  begning of the file). This new fix adds a newline which satisfies both
  regex tests.

- Displayed eqaution blocks at the end of the file were also broken, a
  newline is added to the end to fix this.
Re-remove MathJax LaTeX preprocessor

'tex2jax.js' MathJax preprocessor has been re-removed. It was an
unthought mistake to add it back in; My rules are slightly stricter
than theres for identifying eq blocks, which meant that some equation
blocks could pass through my preprocessor ending up without a
containing `<span class="math">` tag pair, but we processed as LaTeX
by MathJax. This causes problems with the markmon Levenstien algo,
specifically editing such an equation would leave the old render in
the preview, along with the new one.

Its much simpler to say I determine what are equation blocks, and add
MathJax's script tag identifiers manually for its second passes input.