CLI: Remove flake8 dependency, run with pre-commit

[andreoliwa]

Expected Behavior

When running Nitpick, display the correct filenames that contain violations.

Simulate the output of a flake8 plugin, so warnings can be displayed together with tools that use the flake8 format.
If possible, display the correct line and column of the violation.

Also, without relying on flake8, Nitpick can be used for any language, not only for Python projects.
There would be no need to create dummy.py files.

Example output:

$ pre-commit run --all-files nitpick
Nitpick..................................................................Failed
- hook id: nitpick
- exit code: 1

.pre-commit-config.yaml:3:5: NIP339 hook 'debug-statements' (rev: v3.1.0) has different values. Use this:
rev: v3.0.0
.pre-commit-config.yaml:3:5: NIP339 hook 'end-of-file-fixer' (rev: v3.1.0) has different values. Use this:
rev: v3.0.0
.pre-commit-config.yaml:3:5: NIP339 hook 'trailing-whitespace' (rev: v3.1.0) has different values. Use this:
rev: v3.0.0
.pre-commit-config.yaml:9:5: NIP339 hook 'pyupgrade' (rev: v2.4.4) has different values. Use this:
rev: v2.4.1
.pre-commit-config.yaml:50:5: NIP339 hook 'flake8' (rev: 3.8.2) has different values. Use this:
rev: 3.8.1
setup.cfg:25:1: NIP324 section [flake8] has some missing key/value pairs. Use this:
[flake8]
exclude = .tox,build
pyproject.toml:55:1: NIP318 missing values:
[tool.poetry.dev-dependencies]
pylint = "*"

Current Behavior

Nitpick is a flake8 plugin, and flake8 only runs on Python .py files.
Currently, Nitpick chooses an arbitrary Python file and displays all the flake8 warnings as if they were violations of that Python file.
This was mentioned on #73 (comment)

Below is an example of how Nitpick works now.
All violations are displayed under docs/conf.py, line 0, column 1.

$ pre-commit run --all-files nitpick
Nitpick..................................................................Failed
- hook id: nitpick
- exit code: 1

docs/conf.py:0:1: NIP339 File .pre-commit-config.yaml: hook 'debug-statements' (rev: v3.1.0) has different values. Use this:
rev: v3.0.0
docs/conf.py:0:1: NIP339 File .pre-commit-config.yaml: hook 'end-of-file-fixer' (rev: v3.1.0) has different values. Use this:
rev: v3.0.0
docs/conf.py:0:1: NIP339 File .pre-commit-config.yaml: hook 'trailing-whitespace' (rev: v3.1.0) has different values. Use this:
rev: v3.0.0
docs/conf.py:0:1: NIP339 File .pre-commit-config.yaml: hook 'pyupgrade' (rev: v2.4.4) has different values. Use this:
rev: v2.4.1
docs/conf.py:0:1: NIP339 File .pre-commit-config.yaml: hook 'flake8' (rev: 3.8.2) has different values. Use this:
rev: 3.8.1
docs/conf.py:0:1: NIP324 File setup.cfg: section [flake8] has some missing key/value pairs. Use this:
[flake8]
exclude = .tox,build
docs/conf.py:0:1: NIP318 File pyproject.toml has missing values:
[tool.poetry.dev-dependencies]
pylint = "*"

Possible Solution

1. Run a CLI command like nitpick run to check files.
2. Accept zero or more files as arguments.
   Check only the desired files, of check all files if no arguments were provided.
   This would make it compatible with the files attribute on pre-commit.
3. Make flake8 an optional dependency.
   Nitpick would be installed with poetry add --dev --extras flake8 nitpick or pip install nitpick[flake8].
4. Try to parse the violated config file in order to determine the line and column of the violation.