feat: Add --insert flag to enable in-place TOC insertion

[ryo246912]

Fix: #26

This PR implements the --insert flag feature that allows users to insert generated TOC directly into markdown files, similar to the bash version of github-markdown-toc and doctoc.

Supported Commands

# Insert TOC into a file (creates backup by default)
gh-md-toc --insert README.md

# Insert TOC without creating backup
gh-md-toc --insert --no-backup README.md