CriticMarkup and blank lines

[CriticMarkup](http://criticmarkup.com/) is a powerful concept, but does
require a bit of thought.

1. The syntax is orthogonal to that of HTML, and therefore
Markdown/MultiMarkdown. It should really be thought of as a separate "layer"
of markup -- one that needs to be processed *before* parsing the underlying
MultiMarkdown.

2. This leads to additional complexity, as many users would prefer to use
CriticMarkup as an additional feature of MultiMarkdown, that allows displaying
the added/deleted/etc. text in the HTML output(using ``, ``, etc.).
In this instance, CriticMarkup needs to be carefully wrapped around, or
inside, the underlying Markdown -- not "crossing the streams". For example:

This is {++**good**++}.

This is **{++not**++}.

3. Similarly, CriticMarkup becomes difficult to process when used around
newlines. For example, using CriticMarkup to describe the act of splitting an
existing paragraph into two new paragraphs is tricky, as it requires that MMD
"understands" that the opening `{++` in the first paragraph connects to the
closing `++}` in the second paragraph, which violates the normal rules of
Markdown:

This is one para.{++

++} That is now two.

4. An alternative could be to require that such CriticMarkup start and end
with the opening/closing tags on their own separate lines. This could allow
the MMD parser to recognize a CriticMarkup "chunk" and treat it as it's own
block:

This is one para.
{++

++}
That is now two.

5. Another alternative is to create a separate CriticMarkup parsing step that
finds all CriticMarkup and handles accepting or rejecting it, prior to the
regular MMD parsing. This would be similar to what happened in MMD version 5.

For now, CriticMarkup doesn't play well "across" paragraphs in MMD. I will
consider ways of addressing this in the future, and welcome input!