Fix newline handling in Latex double match (issue 674)

[Julynx]

Fixes #674

[Crozzers]

Are you able to add a new test case under test/tm-cases for this?
Usually you just add a few files:

• <testcase name>.text: the markdown that showcases the bug
• <testcase name>.html: the expected bug-free html output
• <testcase name>.opts (optional): any kwargs to pass to the markdown instance, eg: {'extras': ...}

[Crozzers]

Raised #693 with the test case for this PR.

@nicholasserra if you want to merge this then #693 then this issue can be closed