Fix literal block

Summary:
Also try to consolidate lines parser code for literal and code
blocks by adding an intermediate PhutilRemarkupEngineMultiLineEnclosedBlockRule
class.

Test Plan: Tested literal and code blocks in sandbox.

Reviewers: epriestley

Reviewed By: epriestley

CC: ldbrandy, Korvin, aran, wez

Differential Revision: https://secure.phabricator.com/D6848