markdown code blocks escape percent signs

[nteetor]

This problem was first raised in #699.

• My approach is based on that used for inline code
• I added one new test for code blocks and percent signs
• I updated the existing code blocks test to account for the escaped percent signs

[hadley]

Looks great!

Can you please add a bullet to NEWS? It should briefly describe the change and end with (@yourname, #issuenumber).

[nteetor]

Sure thing

[hadley]

Can you please enable edit access for me? https://help.github.com/articles/allowing-changes-to-a-pull-request-branch-created-from-a-fork/#enabling-repository-maintainer-permissions-on-existing-pull-requests

[nteetor]

Sorry, didn't catch up to master. You're all set on editing.

[hadley]

Thanks!