Add diff as a language.

[timvandijck]

As also mentioned in #101 it would be nice to have support for the diff language.

This PR adds support for diff the way it's also used on Github:

+ $this->newCode();
- $this->oldCode();

[brendt]

Perfect PR, nothing to add 👍

[coveralls]

Pull Request Test Coverage Report for Build 8813928833

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 27 of 27 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 94.367%

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Totals
Change from base Build 8783989661:	0.1%
Covered Lines:	1424
Relevant Lines:	1509

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💛 - Coveralls

[brendt]

Tagged: https://github.com/tempestphp/highlight/releases/tag/2.1.0