Bugfix/parsing code error in backticks

[seaoak]

What does it do?

Fix the issue #4190 and #4220.

Current backtick_code_block filter does not consider a Swig template
which form is a pair of triple backticks in single line.

This patch makes the filter to ignore such lines.

And also the filter mistook triple-backtick as the end of targets
even if there are some extra characters at the head of line.

This patch fixes it too.

How to test

git clone -b bugfix/parsing_code_error_in_backticks https://github.com/seaoak/hexo.git
cd hexo
npm install
npm test

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[SukkaW]

What about adding #4190,#4220 related test cases?

[seaoak]

@SukkaW
I only rebase (squash) this branch. Please review.

[SukkaW]

LGTM then! I am going to merge this after I triggering a rebuild at travis.

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SInce Travis CI fail due to their own issue, I am merging this.