Refactor: classes for format checking

[thekaveman]

• Refactors the conventional_pre_commit.format module to use classes instead of the top-level functions
• All existing behavior is now available on instances of either the Commit or ConventionalCommit class
• Refactored all internal code to use these classes instead
• Top-level conventional_pre_commit.format.is_conventional() still exists with the same signature

Finally, this PR introduces the long requested feature to skip merge commits, e.g. Merge branch 'dev' into 'main'. The --strict flag maintains existing behavior of disallowing merge commits.

ht @zlodes #103

Updates the documentation to close #104

[github-actions]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
conventional_pre_commit
format.py
hook.py
output.py					77-78
Project Total

This report was generated by python-coverage-comment-action

[lalver1]

Looks good to me @thekaveman. I went over the refactor to use classes and was able to follow the changes. Then I ran the script to test a merge commit with and without the --strict flag and saw the new behavior.