Linter highlighting

[akaihola]

This also refactors highlighting support into a separate module and uses that in __main__.py for --diff output highlighting.

• Windows build is failing due to Different Black formatting results on Windows #133. Rebase this after merging Fix Black formatting on Windows (issue #133) #142.
• show some screenshots in this PR

[akaihola]

Here's example output from running coverage.py on the test suite and displaying edited lines with no coverage using cov_to_lint.py as the linter:

$ cd ~/darker
$ git checkout 1.0.0
$ coverage run -m pytest -c /dev/null
$ darker --revision 0.1.0.. --check --lint cov_to_lint.py src

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.67%.

Quality metrics	Before	After	Change
Complexity	11.06 🙂	11.03 🙂	-0.03 👍
Method Length	83.24 🙂	83.76 🙂	0.52 👎
Working memory	11.33 😞	11.58 😞	0.25 👎
Quality	54.36% 🙂	53.69% 🙂	-0.67% 👎

Other metrics	Before	After	Change
Lines	853	858	5

Changed files	Quality Before	Quality After	Quality Change
src/darker/main.py	42.35% 😞	42.07% 😞	-0.28% 👎
src/darker/linting.py	63.95% 🙂	60.51% 🙂	-3.44% 👎
src/darker/tests/test_linting.py	77.22% ⭐	77.07% ⭐	-0.15% 👎

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
src/darker/main.py	main	29 😞	406 ⛔	15 😞	19.73% ⛔	Refactor to reduce nesting. Try splitting into smaller methods. Extract out complex expressions
src/darker/main.py	_reformat_single_file	11 🙂	216 ⛔	19 ⛔	35.04% 😞	Try splitting into smaller methods. Extract out complex expressions
src/darker/linting.py	run_linter	13 🙂	140 😞	13 😞	46.43% 😞	Try splitting into smaller methods. Extract out complex expressions
src/darker/tests/test_linting.py	test_run_linter	0 ⭐	247 ⛔	12 😞	51.80% 🙂	Try splitting into smaller methods. Extract out complex expressions
src/darker/main.py	format_edited_parts	10 🙂	78 🙂	11 😞	60.54% 🙂	Extract out complex expressions

Legend and Explanation

The emojis denote the absolute quality of the code:

• ⭐ excellent
• 🙂 good
• 😞 poor
• ⛔ very poor

The 👍 and 👎 indicate whether the quality has improved or gotten worse with this pull request.

---

Please see our documentation here for details on how these metrics are calculated.

We are actively working on this report - lots more documentation and extra metrics to come!

Help us improve this quality report!