Added TOC support (#985)

### Prerequisites

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- [ ] This pull request fixes a bug.
- [x] This pull request adds a feature.
- [ ] This pull request introduces breaking change.

### Description

Adds "Table of Contents" functionality for posts. I believe this was not
implemented before, and I use it on my site.

### Issues Resolved

List any existing issues this pull request resolves.

### Checklist

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#### General

- [x] Describe what changes are being made
- [x] Explain why and how the changes were necessary and implemented
respectively
- [ ] Reference issue with `#<ISSUE_NO>` if applicable

#### Resources

- [ ] If you have changed any SCSS code, run `make release` to
regenerate all CSS files

#### Contributors

- [x] Add yourself to `CONTRIBUTORS.md` if you aren't on it already