Improve parser to detect LineBreak independently of renderer.

When checking if it's a newline preceeded by two spaces, look at the input data rather than the output, since the output depends on the renderer implementation.

Partially fixes #106.

Tests pass.

It should have no effect on the default HTML and Latex parsers, since their `NormalText` method preserves the whitespace. But for 3rd party renderers that do not preserve whitespace, this will fix the problem where `LineBreak` method was not being called despite input Markdown document having a line break (in the form of a newline preceeded by two spaces).