Remove requirement of blank lines for disp eq

At least one blank lines before and after a displayed equation block is
no longer a requirement.
Add support for \[...\] delimiters for display eq

You can in fact use either $$, \[ for the opening delimiter and it will
match to either of $$, \], although it is advised to stick to one
particular format in the same markdown document.
Fix file cannot start with an inline equation bug

If a markdown document started with $...$ without anything preceding it
(including white space) then the document was not parsed correctly.
Update LaTeX instructions for new syntax

Specifically the addition of new delimiters \[...\] for displayed
equations, and not requiring blank lines before and after displayed
equations blocks in the source.